diff --git a/models/issues/comment.go b/models/issues/comment.go
index 34ce7f3500..84a7150b9f 100644
--- a/models/issues/comment.go
+++ b/models/issues/comment.go
@@ -24,6 +24,7 @@ import (
 	"code.gitea.io/gitea/modules/htmlutil"
 	"code.gitea.io/gitea/modules/json"
 	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/markup"
 	"code.gitea.io/gitea/modules/optional"
 	"code.gitea.io/gitea/modules/references"
 	"code.gitea.io/gitea/modules/structs"
@@ -543,6 +544,12 @@ func (c *Comment) EventTag() string {
 	return fmt.Sprintf("event-%d", c.ID)
 }
 
+func (c *Comment) GetSanitizedContentHTML() template.HTML {
+	// mainly for type=4 CommentTypeCommitRef
+	// the content is a link like <a href="{RepoLink}/commit/{CommitID}">message title</a> (from CreateRefComment)
+	return markup.Sanitize(c.Content)
+}
+
 // LoadLabel if comment.Type is CommentTypeLabel, then load Label
 func (c *Comment) LoadLabel(ctx context.Context) error {
 	var label Label
diff --git a/modules/markup/html.go b/modules/markup/html.go
index 1c2ae6918d..0fe37ae305 100644
--- a/modules/markup/html.go
+++ b/modules/markup/html.go
@@ -6,6 +6,7 @@ package markup
 import (
 	"bytes"
 	"fmt"
+	"html/template"
 	"io"
 	"regexp"
 	"slices"
@@ -149,9 +150,9 @@ func PostProcessDefault(ctx *RenderContext, input io.Reader, output io.Writer) e
 	return postProcess(ctx, procs, input, output)
 }
 
-// PostProcessCommitMessage will use the same logic as PostProcess, but will disable
-// the shortLinkProcessor.
-func PostProcessCommitMessage(ctx *RenderContext, content string) (string, error) {
+// PostProcessCommitMessage will use the same logic as PostProcess, but will disable the shortLinkProcessor.
+// FIXME: this function and its family have a very strange design: it takes HTML as input and output, processes the "escaped" content.
+func PostProcessCommitMessage(ctx *RenderContext, content template.HTML) (template.HTML, error) {
 	procs := []processor{
 		fullIssuePatternProcessor,
 		comparePatternProcessor,
@@ -165,7 +166,8 @@ func PostProcessCommitMessage(ctx *RenderContext, content string) (string, error
 		emojiProcessor,
 		emojiShortCodeProcessor,
 	}
-	return postProcessString(ctx, procs, content)
+	s, err := postProcessString(ctx, procs, string(content))
+	return template.HTML(s), err
 }
 
 var emojiProcessors = []processor{
diff --git a/modules/templates/helper.go b/modules/templates/helper.go
index f81be1255a..3e4289c8ad 100644
--- a/modules/templates/helper.go
+++ b/modules/templates/helper.go
@@ -32,13 +32,12 @@ func newFuncMapWebPage() template.FuncMap {
 
 		// -----------------------------------------------------------------
 		// html/template related functions
-		"dict":         dict, // it's lowercase because this name has been widely used. Our other functions should have uppercase names.
-		"Iif":          iif,
-		"Eval":         evalTokens,
-		"HTMLFormat":   htmlFormat,
-		"QueryEscape":  queryEscape,
-		"QueryBuild":   QueryBuild,
-		"SanitizeHTML": SanitizeHTML,
+		"dict":        dict, // it's lowercase because this name has been widely used. Our other functions should have uppercase names.
+		"Iif":         iif,
+		"Eval":        evalTokens,
+		"HTMLFormat":  htmlFormat,
+		"QueryEscape": queryEscape,
+		"QueryBuild":  QueryBuild,
 
 		"PathEscape":         url.PathEscape,
 		"PathEscapeSegments": util.PathEscapeSegments,
@@ -146,9 +145,8 @@ func newFuncMapWebPage() template.FuncMap {
 	}
 }
 
-// SanitizeHTML sanitizes the input by default sanitization rules.
-func SanitizeHTML(s string) template.HTML {
-	return markup.Sanitize(s)
+func sanitizeHTML(msg string) template.HTML {
+	return markup.Sanitize(msg)
 }
 
 func htmlFormat(s any, args ...any) template.HTML {
diff --git a/modules/templates/helper_test.go b/modules/templates/helper_test.go
index f90818c0ad..cf1db32476 100644
--- a/modules/templates/helper_test.go
+++ b/modules/templates/helper_test.go
@@ -58,7 +58,7 @@ func TestSubjectBodySeparator(t *testing.T) {
 }
 
 func TestSanitizeHTML(t *testing.T) {
-	assert.Equal(t, template.HTML(`<a href="/" rel="nofollow">link</a> xss <div>inline</div>`), SanitizeHTML(`<a href="/">link</a> <a href="javascript:">xss</a> <div style="dangerous">inline</div>`))
+	assert.Equal(t, template.HTML(`<a href="/" rel="nofollow">link</a> xss <div>inline</div>`), sanitizeHTML(`<a href="/">link</a> <a href="javascript:">xss</a> <div style="dangerous">inline</div>`))
 }
 
 func TestTemplateIif(t *testing.T) {
diff --git a/modules/templates/mail.go b/modules/templates/mail.go
index 181c6312b0..f81073902f 100644
--- a/modules/templates/mail.go
+++ b/modules/templates/mail.go
@@ -65,13 +65,16 @@ func mailBodyFuncMap() template.FuncMap {
 		"NIL":     func() any { return nil },
 
 		// html/template related functions
-		"dict":         dict,
-		"Iif":          iif,
-		"Eval":         evalTokens,
-		"HTMLFormat":   htmlFormat,
-		"QueryEscape":  queryEscape,
-		"QueryBuild":   QueryBuild,
-		"SanitizeHTML": SanitizeHTML,
+		"dict":        dict,
+		"Iif":         iif,
+		"Eval":        evalTokens,
+		"HTMLFormat":  htmlFormat,
+		"QueryEscape": queryEscape,
+		"QueryBuild":  QueryBuild,
+
+		// deprecated, use "HTMLFormat" instead, but some user custom mail templates still use it
+		// see: https://github.com/go-gitea/gitea/issues/36049
+		"SanitizeHTML": sanitizeHTML,
 
 		"PathEscape":         url.PathEscape,
 		"PathEscapeSegments": util.PathEscapeSegments,
diff --git a/modules/templates/util_render.go b/modules/templates/util_render.go
index 081a13fb9e..6641ee7959 100644
--- a/modules/templates/util_render.go
+++ b/modules/templates/util_render.go
@@ -40,7 +40,7 @@ func NewRenderUtils(ctx reqctx.RequestContext) *RenderUtils {
 
 // RenderCommitMessage renders commit message with XSS-safe and special links.
 func (ut *RenderUtils) RenderCommitMessage(msg string, repo *repo.Repository) template.HTML {
-	cleanMsg := template.HTMLEscapeString(msg)
+	cleanMsg := template.HTML(template.HTMLEscapeString(msg))
 	// we can safely assume that it will not return any error, since there shouldn't be any special HTML.
 	// "repo" can be nil when rendering commit messages for deleted repositories in a user's dashboard feed.
 	fullMessage, err := markup.PostProcessCommitMessage(renderhelper.NewRenderContextRepoComment(ut.ctx, repo), cleanMsg)
@@ -48,7 +48,7 @@ func (ut *RenderUtils) RenderCommitMessage(msg string, repo *repo.Repository) te
 		log.Error("PostProcessCommitMessage: %v", err)
 		return ""
 	}
-	msgLines := strings.Split(strings.TrimSpace(fullMessage), "\n")
+	msgLines := strings.Split(strings.TrimSpace(string(fullMessage)), "\n")
 	if len(msgLines) == 0 {
 		return ""
 	}
@@ -91,12 +91,14 @@ func (ut *RenderUtils) RenderCommitBody(msg string, repo *repo.Repository) templ
 		return ""
 	}
 
-	renderedMessage, err := markup.PostProcessCommitMessage(renderhelper.NewRenderContextRepoComment(ut.ctx, repo), template.HTMLEscapeString(msgLine))
+	rctx := renderhelper.NewRenderContextRepoComment(ut.ctx, repo)
+	htmlContent := template.HTML(template.HTMLEscapeString(msgLine))
+	renderedMessage, err := markup.PostProcessCommitMessage(rctx, htmlContent)
 	if err != nil {
 		log.Error("PostProcessCommitMessage: %v", err)
 		return ""
 	}
-	return template.HTML(renderedMessage)
+	return renderedMessage
 }
 
 // Match text that is between back ticks.
@@ -279,6 +281,35 @@ func (ut *RenderUtils) RenderThemeItem(info *webtheme.ThemeMetaInfo, iconSize in
 	return htmlutil.HTMLFormat(`<div class="theme-menu-item" data-tooltip-content="%s">%s %s %s</div>`, info.GetDescription(), icon, info.DisplayName, extraIcon)
 }
 
+func (ut *RenderUtils) RenderFlashMessage(typ, msg string) template.HTML {
+	msg = strings.TrimSpace(msg)
+	if msg == "" {
+		return ""
+	}
+
+	cls := typ
+	// legacy logic: "negative" for error, "positive" for success
+	switch cls {
+	case "error":
+		cls = "negative"
+	case "success":
+		cls = "positive"
+	}
+
+	var msgContent template.HTML
+	if strings.Contains(msg, "</pre>") || strings.Contains(msg, "</details>") || strings.Contains(msg, "</ul>") || strings.Contains(msg, "</div>") {
+		// If the message contains some known "block" elements, no need to do more alignment or line-break processing, just sanitize it directly.
+		msgContent = sanitizeHTML(msg)
+	} else if !strings.Contains(msg, "\n") {
+		// If the message is a single line, center-align it by wrapping it
+		msgContent = htmlutil.HTMLFormat(`<div class="tw-text-center">%s</div>`, sanitizeHTML(msg))
+	} else {
+		// For a multi-line message, preserve line breaks, and left-align it.
+		msgContent = htmlutil.HTMLFormat(`%s`, sanitizeHTML(strings.ReplaceAll(msg, "\n", "<br>")))
+	}
+	return htmlutil.HTMLFormat(`<div class="ui %s message flash-message flash-%s">%s</div>`, cls, typ, msgContent)
+}
+
 func (ut *RenderUtils) RenderUnicodeEscapeToggleButton(escapeStatus *charset.EscapeStatus) template.HTML {
 	if escapeStatus == nil || !escapeStatus.Escaped {
 		return ""
diff --git a/routers/utils/utils.go b/routers/utils/utils.go
index 3035073d5c..47ca13b2aa 100644
--- a/routers/utils/utils.go
+++ b/routers/utils/utils.go
@@ -5,10 +5,11 @@ package utils
 
 import (
 	"html"
-	"strings"
+	"html/template"
 )
 
-// SanitizeFlashErrorString will sanitize a flash error string
-func SanitizeFlashErrorString(x string) string {
-	return strings.ReplaceAll(html.EscapeString(x), "\n", "<br>")
+// EscapeFlashErrorString will escape the flash error string
+// Maybe do more sanitization in the future, e.g.: hide sensitive information, etc.
+func EscapeFlashErrorString(x string) template.HTML {
+	return template.HTML(html.EscapeString(x))
 }
diff --git a/routers/utils/utils_test.go b/routers/utils/utils_test.go
index cc7c888a75..5dfc544777 100644
--- a/routers/utils/utils_test.go
+++ b/routers/utils/utils_test.go
@@ -4,16 +4,17 @@
 package utils
 
 import (
+	"html/template"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
 )
 
-func TestSanitizeFlashErrorString(t *testing.T) {
+func TestEscapeFlashErrorString(t *testing.T) {
 	tests := []struct {
 		name string
 		arg  string
-		want string
+		want template.HTML
 	}{
 		{
 			name: "no error",
@@ -28,13 +29,13 @@ func TestSanitizeFlashErrorString(t *testing.T) {
 		{
 			name: "line break error",
 			arg:  "some error:\n\nawesome!",
-			want: "some error:<br><br>awesome!",
+			want: "some error:\n\nawesome!",
 		},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			got := SanitizeFlashErrorString(tt.arg)
+			got := EscapeFlashErrorString(tt.arg)
 			assert.Equal(t, tt.want, got)
 		})
 	}
diff --git a/routers/web/auth/oauth.go b/routers/web/auth/oauth.go
index c103d20064..8645aedbde 100644
--- a/routers/web/auth/oauth.go
+++ b/routers/web/auth/oauth.go
@@ -79,7 +79,7 @@ func SignInOAuthCallback(ctx *context.Context) {
 			}
 		}
 		sort.Strings(errorKeyValues)
-		ctx.Flash.Error(strings.Join(errorKeyValues, "<br>"), true)
+		ctx.Flash.Error(strings.Join(errorKeyValues, "\n"), true)
 	}
 
 	// first look if the provider is still active
diff --git a/routers/web/devtest/devtest.go b/routers/web/devtest/devtest.go
index 5cfe08c7fe..8bc5947df8 100644
--- a/routers/web/devtest/devtest.go
+++ b/routers/web/devtest/devtest.go
@@ -45,8 +45,8 @@ func List(ctx *context.Context) {
 
 func FetchActionTest(ctx *context.Context) {
 	_ = ctx.Req.ParseForm()
-	ctx.Flash.Info("fetch-action: " + ctx.Req.Method + " " + ctx.Req.RequestURI + "<br>" +
-		"Form: " + ctx.Req.Form.Encode() + "<br>" +
+	ctx.Flash.Info("fetch-action: " + ctx.Req.Method + " " + ctx.Req.RequestURI + "\n" +
+		"Form: " + ctx.Req.Form.Encode() + "\n" +
 		"PostForm: " + ctx.Req.PostForm.Encode(),
 	)
 	time.Sleep(2 * time.Second)
@@ -192,11 +192,31 @@ func prepareMockData(ctx *context.Context) {
 		prepareMockDataBadgeActionsSvg(ctx)
 	case "/devtest/relative-time":
 		prepareMockDataRelativeTime(ctx)
+	case "/devtest/toast-and-message":
+		prepareMockDataToastAndMessage(ctx)
 	case "/devtest/unicode-escape":
 		prepareMockDataUnicodeEscape(ctx)
 	}
 }
 
+func prepareMockDataToastAndMessage(ctx *context.Context) {
+	msgWithDetails, _ := ctx.RenderToHTML("base/alert_details", map[string]any{
+		"Message": "message with details <script>escape xss</script>",
+		"Summary": "summary with details",
+		"Details": "details line 1\n details line 2\n details line 3",
+	})
+	msgWithSummary, _ := ctx.RenderToHTML("base/alert_details", map[string]any{
+		"Message": "message with summary <script>escape xss</script>",
+		"Summary": "summary only",
+	})
+
+	ctx.Flash.ErrorMsg = string(msgWithDetails)
+	ctx.Flash.WarningMsg = string(msgWithSummary)
+	ctx.Flash.InfoMsg = "a long message with line break\nthe second line <script>removed xss</script>"
+	ctx.Flash.SuccessMsg = "single line message <script>removed xss</script>"
+	ctx.Data["Flash"] = ctx.Flash
+}
+
 func prepareMockDataUnicodeEscape(ctx *context.Context) {
 	content := "// demo code\n"
 	content += "if accessLevel != \"user\u202E \u2066// Check if admin (invisible char)\u2069 \u2066\" { }\n"
@@ -223,8 +243,8 @@ func TmplCommon(ctx *context.Context) {
 	prepareMockData(ctx)
 	if ctx.Req.Method == http.MethodPost {
 		_ = ctx.Req.ParseForm()
-		ctx.Flash.Info("form: "+ctx.Req.Method+" "+ctx.Req.RequestURI+"<br>"+
-			"Form: "+ctx.Req.Form.Encode()+"<br>"+
+		ctx.Flash.Info("form: "+ctx.Req.Method+" "+ctx.Req.RequestURI+"\n"+
+			"Form: "+ctx.Req.Form.Encode()+"\n"+
 			"PostForm: "+ctx.Req.PostForm.Encode(),
 			true,
 		)
diff --git a/routers/web/feed/convert.go b/routers/web/feed/convert.go
index a5c379e01a..5d208bb286 100644
--- a/routers/web/feed/convert.go
+++ b/routers/web/feed/convert.go
@@ -15,6 +15,7 @@ import (
 	activities_model "code.gitea.io/gitea/models/activities"
 	"code.gitea.io/gitea/models/renderhelper"
 	repo_model "code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/modules/markup"
 	"code.gitea.io/gitea/modules/markup/markdown"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/templates"
@@ -237,7 +238,7 @@ func feedActionsToFeedItems(ctx *context.Context, actions activities_model.Actio
 			}
 		}
 		if len(content) == 0 {
-			content = templates.SanitizeHTML(desc)
+			content = markup.Sanitize(desc)
 		}
 
 		items = append(items, &feeds.Item{
diff --git a/routers/web/repo/branch.go b/routers/web/repo/branch.go
index 5e5cfec5c2..c566e465e9 100644
--- a/routers/web/repo/branch.go
+++ b/routers/web/repo/branch.go
@@ -231,7 +231,7 @@ func CreateBranch(ctx *context.Context) {
 				flashError, err := ctx.RenderToHTML(tplAlertDetails, map[string]any{
 					"Message": ctx.Tr("repo.editor.push_rejected"),
 					"Summary": ctx.Tr("repo.editor.push_rejected_summary"),
-					"Details": utils.SanitizeFlashErrorString(e.Message),
+					"Details": utils.EscapeFlashErrorString(e.Message),
 				})
 				if err != nil {
 					ctx.ServerError("UpdatePullRequest.HTMLString", err)
diff --git a/routers/web/repo/commit.go b/routers/web/repo/commit.go
index 168d959494..34e588b141 100644
--- a/routers/web/repo/commit.go
+++ b/routers/web/repo/commit.go
@@ -410,7 +410,8 @@ func Diff(ctx *context.Context) {
 		ctx.Data["NoteCommit"] = note.Commit
 		ctx.Data["NoteAuthor"] = user_model.ValidateCommitWithEmail(ctx, note.Commit)
 		rctx := renderhelper.NewRenderContextRepoComment(ctx, ctx.Repo.Repository, renderhelper.RepoCommentOptions{CurrentRefPath: path.Join("commit", util.PathEscapeSegments(commitID))})
-		ctx.Data["NoteRendered"], err = markup.PostProcessCommitMessage(rctx, template.HTMLEscapeString(string(charset.ToUTF8WithFallback(note.Message, charset.ConvertOpts{}))))
+		htmlMessage := template.HTML(template.HTMLEscapeString(string(charset.ToUTF8WithFallback(note.Message, charset.ConvertOpts{}))))
+		ctx.Data["NoteRendered"], err = markup.PostProcessCommitMessage(rctx, htmlMessage)
 		if err != nil {
 			ctx.ServerError("PostProcessCommitMessage", err)
 			return
diff --git a/routers/web/repo/editor_error.go b/routers/web/repo/editor_error.go
index e1473a34b3..f23b2738e5 100644
--- a/routers/web/repo/editor_error.go
+++ b/routers/web/repo/editor_error.go
@@ -27,13 +27,13 @@ func editorHandleFileOperationErrorRender(ctx *context_service.Context, message,
 	flashError, err := ctx.RenderToHTML(tplAlertDetails, map[string]any{
 		"Message": message,
 		"Summary": summary,
-		"Details": utils.SanitizeFlashErrorString(details),
+		"Details": utils.EscapeFlashErrorString(details),
 	})
 	if err == nil {
 		ctx.JSONError(flashError)
 	} else {
-		log.Error("RenderToHTML: %v", err)
-		ctx.JSONError(message + "\n" + summary + "\n" + utils.SanitizeFlashErrorString(details))
+		log.Error("RenderToHTML(%q, %q, %q), error: %v", message, summary, details, err)
+		ctx.JSONError("Unable to render error details, see server logs") // it should never happen
 	}
 }
 
diff --git a/routers/web/repo/issue_new.go b/routers/web/repo/issue_new.go
index 98fb842ddf..592d902ba8 100644
--- a/routers/web/repo/issue_new.go
+++ b/routers/web/repo/issue_new.go
@@ -170,7 +170,7 @@ func renderErrorOfTemplates(ctx *context.Context, errs map[string]error) templat
 	flashError, err := ctx.RenderToHTML(tplAlertDetails, map[string]any{
 		"Message": ctx.Tr("repo.issues.choose.ignore_invalid_templates"),
 		"Summary": ctx.Tr("repo.issues.choose.invalid_templates", len(errs)),
-		"Details": utils.SanitizeFlashErrorString(strings.Join(lines, "\n")),
+		"Details": utils.EscapeFlashErrorString(strings.Join(lines, "\n")),
 	})
 	if err != nil {
 		log.Debug("render flash error: %v", err)
diff --git a/routers/web/repo/issue_view.go b/routers/web/repo/issue_view.go
index 250a54fc24..f678f83878 100644
--- a/routers/web/repo/issue_view.go
+++ b/routers/web/repo/issue_view.go
@@ -29,7 +29,6 @@ import (
 	"code.gitea.io/gitea/modules/markup"
 	"code.gitea.io/gitea/modules/markup/markdown"
 	"code.gitea.io/gitea/modules/setting"
-	"code.gitea.io/gitea/modules/templates"
 	"code.gitea.io/gitea/modules/templates/vars"
 	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/modules/web/middleware"
@@ -781,14 +780,14 @@ func prepareIssueViewCommentsAndSidebarParticipants(ctx *context.Context, issue
 		} else if comment.Type == issues_model.CommentTypeAddTimeManual ||
 			comment.Type == issues_model.CommentTypeStopTracking ||
 			comment.Type == issues_model.CommentTypeDeleteTimeManual {
-			// drop error since times could be pruned from DB..
+			// drop error since times could be pruned from DB
 			_ = comment.LoadTime(ctx)
 			if comment.Content != "" {
 				// Content before v1.21 did store the formatted string instead of seconds,
 				// so "|" is used as delimiter to mark the new format
 				if comment.Content[0] != '|' {
 					// handle old time comments that have formatted text stored
-					comment.RenderedContent = templates.SanitizeHTML(comment.Content)
+					comment.RenderedContent = markup.Sanitize(comment.Content)
 					comment.Content = ""
 				} else {
 					// else it's just a duration in seconds to pass on to the frontend
diff --git a/routers/web/repo/pull.go b/routers/web/repo/pull.go
index e312fc9d2a..efcdaac674 100644
--- a/routers/web/repo/pull.go
+++ b/routers/web/repo/pull.go
@@ -1042,7 +1042,7 @@ func UpdatePullRequest(ctx *context.Context) {
 			flashError, err := ctx.RenderToHTML(tplAlertDetails, map[string]any{
 				"Message": ctx.Tr("repo.pulls.merge_conflict"),
 				"Summary": ctx.Tr("repo.pulls.merge_conflict_summary"),
-				"Details": utils.SanitizeFlashErrorString(conflictError.StdErr) + "<br>" + utils.SanitizeFlashErrorString(conflictError.StdOut),
+				"Details": utils.EscapeFlashErrorString(conflictError.StdErr) + "\n" + utils.EscapeFlashErrorString(conflictError.StdOut),
 			})
 			if err != nil {
 				ctx.ServerError("UpdatePullRequest.HTMLString", err)
@@ -1054,9 +1054,9 @@ func UpdatePullRequest(ctx *context.Context) {
 		} else if pull_service.IsErrRebaseConflicts(err) {
 			conflictError := err.(pull_service.ErrRebaseConflicts)
 			flashError, err := ctx.RenderToHTML(tplAlertDetails, map[string]any{
-				"Message": ctx.Tr("repo.pulls.rebase_conflict", utils.SanitizeFlashErrorString(conflictError.CommitSHA)),
+				"Message": ctx.Tr("repo.pulls.rebase_conflict", utils.EscapeFlashErrorString(conflictError.CommitSHA)),
 				"Summary": ctx.Tr("repo.pulls.rebase_conflict_summary"),
-				"Details": utils.SanitizeFlashErrorString(conflictError.StdErr) + "<br>" + utils.SanitizeFlashErrorString(conflictError.StdOut),
+				"Details": utils.EscapeFlashErrorString(conflictError.StdErr) + "\n" + utils.EscapeFlashErrorString(conflictError.StdOut),
 			})
 			if err != nil {
 				ctx.ServerError("UpdatePullRequest.HTMLString", err)
@@ -1191,7 +1191,7 @@ func MergePullRequest(ctx *context.Context) {
 			flashError, err := ctx.RenderToHTML(tplAlertDetails, map[string]any{
 				"Message": ctx.Tr("repo.editor.merge_conflict"),
 				"Summary": ctx.Tr("repo.editor.merge_conflict_summary"),
-				"Details": utils.SanitizeFlashErrorString(conflictError.StdErr) + "<br>" + utils.SanitizeFlashErrorString(conflictError.StdOut),
+				"Details": utils.EscapeFlashErrorString(conflictError.StdErr) + "\n" + utils.EscapeFlashErrorString(conflictError.StdOut),
 			})
 			if err != nil {
 				ctx.ServerError("MergePullRequest.HTMLString", err)
@@ -1202,9 +1202,9 @@ func MergePullRequest(ctx *context.Context) {
 		} else if pull_service.IsErrRebaseConflicts(err) {
 			conflictError := err.(pull_service.ErrRebaseConflicts)
 			flashError, err := ctx.RenderToHTML(tplAlertDetails, map[string]any{
-				"Message": ctx.Tr("repo.pulls.rebase_conflict", utils.SanitizeFlashErrorString(conflictError.CommitSHA)),
+				"Message": ctx.Tr("repo.pulls.rebase_conflict", utils.EscapeFlashErrorString(conflictError.CommitSHA)),
 				"Summary": ctx.Tr("repo.pulls.rebase_conflict_summary"),
-				"Details": utils.SanitizeFlashErrorString(conflictError.StdErr) + "<br>" + utils.SanitizeFlashErrorString(conflictError.StdOut),
+				"Details": utils.EscapeFlashErrorString(conflictError.StdErr) + "\n" + utils.EscapeFlashErrorString(conflictError.StdOut),
 			})
 			if err != nil {
 				ctx.ServerError("MergePullRequest.HTMLString", err)
@@ -1234,7 +1234,7 @@ func MergePullRequest(ctx *context.Context) {
 				flashError, err := ctx.RenderToHTML(tplAlertDetails, map[string]any{
 					"Message": ctx.Tr("repo.pulls.push_rejected"),
 					"Summary": ctx.Tr("repo.pulls.push_rejected_summary"),
-					"Details": utils.SanitizeFlashErrorString(pushrejErr.Message),
+					"Details": utils.EscapeFlashErrorString(pushrejErr.Message),
 				})
 				if err != nil {
 					ctx.ServerError("MergePullRequest.HTMLString", err)
@@ -1454,7 +1454,7 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 			flashError, err := ctx.RenderToHTML(tplAlertDetails, map[string]any{
 				"Message": ctx.Tr("repo.pulls.push_rejected"),
 				"Summary": ctx.Tr("repo.pulls.push_rejected_summary"),
-				"Details": utils.SanitizeFlashErrorString(pushrejErr.Message),
+				"Details": utils.EscapeFlashErrorString(pushrejErr.Message),
 			})
 			if err != nil {
 				ctx.ServerError("CompareAndPullRequest.HTMLString", err)
diff --git a/templates/base/alert.tmpl b/templates/base/alert.tmpl
index 5ebe191771..242f6278ea 100644
--- a/templates/base/alert.tmpl
+++ b/templates/base/alert.tmpl
@@ -1,25 +1,9 @@
-{{- if .Flash.ErrorMsg -}}
-	<div class="ui negative message flash-message flash-error">
-		<p>{{.Flash.ErrorMsg | SanitizeHTML}}</p>
-	</div>
-{{- end -}}
-{{- if .Flash.SuccessMsg -}}
-	<div class="ui positive message flash-message flash-success">
-		<p>{{.Flash.SuccessMsg | SanitizeHTML}}</p>
-	</div>
-{{- end -}}
-{{- if .Flash.InfoMsg -}}
-	<div class="ui info message flash-message flash-info">
-		<p>{{.Flash.InfoMsg | SanitizeHTML}}</p>
-	</div>
-{{- end -}}
-{{- if .Flash.WarningMsg -}}
-	<div class="ui warning message flash-message flash-warning">
-		<p>{{.Flash.WarningMsg | SanitizeHTML}}</p>
-	</div>
-{{- end -}}
+{{- if .Flash.ErrorMsg}}{{ctx.RenderUtils.RenderFlashMessage "error" .Flash.ErrorMsg}}{{end -}}
+{{- if .Flash.WarningMsg}}{{ctx.RenderUtils.RenderFlashMessage "warning" .Flash.WarningMsg}}{{end -}}
+{{- if .Flash.InfoMsg}}{{ctx.RenderUtils.RenderFlashMessage "info" .Flash.InfoMsg}}{{end -}}
+{{- if .Flash.SuccessMsg}}{{ctx.RenderUtils.RenderFlashMessage "success" .Flash.SuccessMsg}}{{end -}}
 {{- if .ShowTwoFactorRequiredMessage -}}
-<div class="ui negative message flash-message flash-error">
-	<p><a href="{{AppSubUrl}}/user/settings/security/two_factor/enroll">{{ctx.Locale.Tr "auth.twofa_required"}}</a></p>
+<div class="ui error message flash-message flash-error">
+	<a href="{{AppSubUrl}}/user/settings/security/two_factor/enroll">{{ctx.Locale.Tr "auth.twofa_required"}}</a>
 </div>
 {{- end -}}
diff --git a/templates/base/alert_details.tmpl b/templates/base/alert_details.tmpl
index 6380a72498..da8aba452a 100644
--- a/templates/base/alert_details.tmpl
+++ b/templates/base/alert_details.tmpl
@@ -2,10 +2,8 @@
 {{if .Details}}
 <details>
 	<summary>{{.Summary}}</summary>
-	{{.Details | SanitizeHTML}}
+	<pre>{{.Details}}</pre>
 </details>
 {{else}}
-<div>
-	{{.Summary}}
-</div>
+<div>{{.Summary}}</div>
 {{end}}
diff --git a/templates/devtest/devtest-header.tmpl b/templates/devtest/devtest-header.tmpl
index 628e4388a0..c9d7b3047f 100644
--- a/templates/devtest/devtest-header.tmpl
+++ b/templates/devtest/devtest-header.tmpl
@@ -1,4 +1,4 @@
 {{template "base/head" ctx.RootData}}
 <link rel="stylesheet" href="{{AssetURI "css/devtest.css"}}">
 <div class="tw-hidden" data-global-init="initDevtestPage"></div>
-{{template "base/alert" .}}
+<div class="ui container tw-mt-4">{{template "base/alert" ctx.RootData}}</div>
diff --git a/templates/devtest/fetch-action.tmpl b/templates/devtest/fetch-action.tmpl
index 4ee824f04b..cd4da52aac 100644
--- a/templates/devtest/fetch-action.tmpl
+++ b/templates/devtest/fetch-action.tmpl
@@ -1,6 +1,5 @@
 {{template "devtest/devtest-header"}}
 <div class="page-content devtest ui container">
-	{{template "base/alert" .}}
 	<div>
 		<h1>link-action</h1>
 		<div>
@@ -17,29 +16,20 @@
 	<div>
 		<h1>form-fetch-action</h1>
 		<div>Use "window.fetch" to send a form request to backend</div>
-		<div>
-			<form method="get" action="fetch-action-test?k=1" class="form-fetch-action">
+		<div class="flex-relaxed-list fetch-action-demo-forms">
+			<form method="get" action="./fetch-action-test?k=1" class="form-fetch-action">
 				<button name="btn">submit get</button>
 			</form>
-			<form method="post" action="fetch-action-test?k=1" class="form-fetch-action">
+			<form method="post" action="./fetch-action-test?k=1" class="form-fetch-action">
 				<div><textarea name="text" rows="3"></textarea></div>
 				<div><label><input name="check" type="checkbox"> check</label></div>
 				<div><button name="btn">submit post</button></div>
 			</form>
-			<form method="post" action="no-such-uri" class="form-fetch-action">
+			<form method="post" action="./no-such-uri" class="form-fetch-action">
 				<div class="tw-py-8">bad action url</div>
 				<div><button name="btn">submit test</button></div>
 			</form>
 		</div>
 	</div>
 </div>
-<style>
-	.ui.message.flash-message {
-		text-align: left;
-	}
-	.form-fetch-action {
-		margin-bottom: 1em;
-		border: 1px red dashed; /* show the border for demo purpose */
-	}
-</style>
 {{template "devtest/devtest-footer"}}
diff --git a/templates/devtest/toast.tmpl b/templates/devtest/toast-and-message.tmpl
similarity index 96%
rename from templates/devtest/toast.tmpl
rename to templates/devtest/toast-and-message.tmpl
index 597b415469..c4056b6fc6 100644
--- a/templates/devtest/toast.tmpl
+++ b/templates/devtest/toast-and-message.tmpl
@@ -1,5 +1,5 @@
 {{template "devtest/devtest-header"}}
-<div>
+<div class="ui container">
 	<h1>Toast</h1>
 	<div>
 		<button class="ui button toast-test-button" data-toast-level="info" data-toast-message="test info">Show Info Toast</button>
diff --git a/templates/repo/commit_page.tmpl b/templates/repo/commit_page.tmpl
index 179f6018d3..8451a6f3cf 100644
--- a/templates/repo/commit_page.tmpl
+++ b/templates/repo/commit_page.tmpl
@@ -195,7 +195,7 @@
 				<span class="tw-text-text-light">{{DateUtils.TimeSince .NoteCommit.Author.When}}</span>
 			</div>
 			<div class="ui bottom attached info segment git-notes">
-				<pre class="commit-body">{{.NoteRendered | SanitizeHTML}}</pre>
+				<pre class="commit-body">{{.NoteRendered}}</pre>
 			</div>
 		{{end}}
 
diff --git a/templates/repo/issue/view_content/comments.tmpl b/templates/repo/issue/view_content/comments.tmpl
index 8c08f1c1f1..6a67ef754d 100644
--- a/templates/repo/issue/view_content/comments.tmpl
+++ b/templates/repo/issue/view_content/comments.tmpl
@@ -164,7 +164,7 @@
 				<div class="detail flex-text-block">
 					{{svg "octicon-git-commit"}}
 					{{/* the content is a link like <a href="{RepoLink}/commit/{CommitID}">message title</a> (from CreateRefComment) */}}
-					<span class="comment-text-line">{{.Content | SanitizeHTML}}</span>
+					<span class="comment-text-line">{{.GetSanitizedContentHTML}}</span>
 				</div>
 			</div>
 		{{else if eq .Type 7}}
diff --git a/tests/integration/pull_merge_test.go b/tests/integration/pull_merge_test.go
index 53709e6ff4..d0213962ee 100644
--- a/tests/integration/pull_merge_test.go
+++ b/tests/integration/pull_merge_test.go
@@ -317,9 +317,8 @@ func TestPullCleanUpAfterMerge(t *testing.T) {
 		resp = session.MakeRequest(t, req, http.StatusOK)
 
 		htmlDoc := NewHTMLParser(t, resp.Body)
-		resultMsg := htmlDoc.doc.Find(".ui.message>p").Text()
-
-		assert.Equal(t, "Branch \"user1/repo1:feature/test\" has been deleted.", resultMsg)
+		resultMsg := strings.TrimSpace(htmlDoc.doc.Find(".ui.message.flash-message").Text())
+		assert.Equal(t, `Branch "user1/repo1:feature/test" has been deleted.`, resultMsg)
 	})
 }
 
diff --git a/tests/integration/signin_test.go b/tests/integration/signin_test.go
index ff35baae9d..4000c7ebe1 100644
--- a/tests/integration/signin_test.go
+++ b/tests/integration/signin_test.go
@@ -36,8 +36,7 @@ func testLoginFailed(t *testing.T, username, password, message string) {
 	resp := session.MakeRequest(t, req, http.StatusOK)
 
 	htmlDoc := NewHTMLParser(t, resp.Body)
-	resultMsg := htmlDoc.doc.Find(".ui.message>p").Text()
-
+	resultMsg := strings.TrimSpace(htmlDoc.doc.Find(".ui.message.flash-message").Text())
 	assert.Equal(t, message, resultMsg)
 }
 
diff --git a/tests/integration/user_settings_test.go b/tests/integration/user_settings_test.go
index 20c758dc85..b3527dd467 100644
--- a/tests/integration/user_settings_test.go
+++ b/tests/integration/user_settings_test.go
@@ -5,6 +5,7 @@ package integration
 
 import (
 	"net/http"
+	"strings"
 	"testing"
 
 	"code.gitea.io/gitea/modules/container"
@@ -309,8 +310,7 @@ func TestUserSettingsApplications(t *testing.T) {
 				})
 				resp := session.MakeRequest(t, req, http.StatusOK)
 				doc := NewHTMLParser(t, resp.Body)
-
-				msg := doc.Find(".flash-error p").Text()
+				msg := strings.TrimSpace(doc.Find(".ui.message.flash-message").Text())
 				assert.Equal(t, `form.RedirectURIs"ftp://127.0.0.1" is not a valid URL.`, msg)
 			})
 
diff --git a/web_src/css/base.css b/web_src/css/base.css
index c278132a00..4448af68e0 100644
--- a/web_src/css/base.css
+++ b/web_src/css/base.css
@@ -431,8 +431,9 @@ img.ui.avatar,
   margin-top: calc(var(--page-spacing) - 1rem);
 }
 
-.ui .message.flash-message {
-  text-align: center;
+.ui.message.flash-message pre {
+  white-space: pre-line;
+  margin: 0;
 }
 
 .ui .header > i + .content {
@@ -865,6 +866,13 @@ table th[data-sortt-desc] .svg {
   align-items: stretch;
 }
 
+/* can be used to replace "ui relaxed list" or "tw-flex tw-flex-col tw-gap-xxx" when we need more flexible layout */
+.flex-relaxed-list {
+  display: flex;
+  flex-direction: column;
+  gap: var(--gap-block);
+}
+
 .ui.list.flex-items-block > .item,
 .ui.vertical.menu.flex-items-block > .item,
 .ui.form .field > label.flex-text-block, /* override fomantic "block" style */
diff --git a/web_src/css/devtest.css b/web_src/css/devtest.css
index a7b00e1e56..c344d99058 100644
--- a/web_src/css/devtest.css
+++ b/web_src/css/devtest.css
@@ -1,3 +1,8 @@
+h1, h2 {
+  margin: 0;
+  padding: 10px 0;
+}
+
 .button-sample-groups {
   margin: 0; padding: 0;
 }
@@ -10,7 +15,6 @@
   margin-bottom: 5px;
 }
 
-h1, h2 {
-  margin: 0;
-  padding: 10px 0;
+.fetch-action-demo-forms .form-fetch-action {
+  border: 1px red dashed; /* show the border for demo purpose */
 }