diff --git a/packages/ckeditor5-admonition/tsconfig.json b/packages/ckeditor5-admonition/tsconfig.json
index c239ab5c98..303791fc66 100644
--- a/packages/ckeditor5-admonition/tsconfig.json
+++ b/packages/ckeditor5-admonition/tsconfig.json
@@ -26,6 +26,7 @@
 	"moduleResolution": "NodeNext",
 	"module": "NodeNext",
 	"skipLibCheck": true,
+	"noUncheckedSideEffectImports": false,
 	"outDir": "out-tsc",
     "typeRoots": [
       "typings",
diff --git a/packages/ckeditor5-footnotes/tsconfig.json b/packages/ckeditor5-footnotes/tsconfig.json
index c239ab5c98..303791fc66 100644
--- a/packages/ckeditor5-footnotes/tsconfig.json
+++ b/packages/ckeditor5-footnotes/tsconfig.json
@@ -26,6 +26,7 @@
 	"moduleResolution": "NodeNext",
 	"module": "NodeNext",
 	"skipLibCheck": true,
+	"noUncheckedSideEffectImports": false,
 	"outDir": "out-tsc",
     "typeRoots": [
       "typings",
diff --git a/packages/ckeditor5-keyboard-marker/tsconfig.json b/packages/ckeditor5-keyboard-marker/tsconfig.json
index 089f1440a9..4248cd9589 100644
--- a/packages/ckeditor5-keyboard-marker/tsconfig.json
+++ b/packages/ckeditor5-keyboard-marker/tsconfig.json
@@ -26,6 +26,7 @@
 	"moduleResolution": "NodeNext",
 	"module": "NodeNext",
 	"skipLibCheck": true,
+	"noUncheckedSideEffectImports": false,
 	"outDir": "out-tsc",
     "typeRoots": [
       "typings",
diff --git a/packages/ckeditor5-math/tsconfig.json b/packages/ckeditor5-math/tsconfig.json
index c239ab5c98..303791fc66 100644
--- a/packages/ckeditor5-math/tsconfig.json
+++ b/packages/ckeditor5-math/tsconfig.json
@@ -26,6 +26,7 @@
 	"moduleResolution": "NodeNext",
 	"module": "NodeNext",
 	"skipLibCheck": true,
+	"noUncheckedSideEffectImports": false,
 	"outDir": "out-tsc",
     "typeRoots": [
       "typings",
diff --git a/packages/ckeditor5-mermaid/tsconfig.json b/packages/ckeditor5-mermaid/tsconfig.json
index c239ab5c98..303791fc66 100644
--- a/packages/ckeditor5-mermaid/tsconfig.json
+++ b/packages/ckeditor5-mermaid/tsconfig.json
@@ -26,6 +26,7 @@
 	"moduleResolution": "NodeNext",
 	"module": "NodeNext",
 	"skipLibCheck": true,
+	"noUncheckedSideEffectImports": false,
 	"outDir": "out-tsc",
     "typeRoots": [
       "typings",
diff --git a/packages/ckeditor5/tsconfig.lib.json b/packages/ckeditor5/tsconfig.lib.json
index 00ee5d2e1d..748d57ae0b 100644
--- a/packages/ckeditor5/tsconfig.lib.json
+++ b/packages/ckeditor5/tsconfig.lib.json
@@ -3,7 +3,6 @@
   "compilerOptions": {
     "module": "ESNext",
     "moduleResolution": "bundler",
-    "baseUrl": ".",
     "rootDir": "src",
     "outDir": "dist",
     "tsBuildInfoFile": "dist/tsconfig.lib.tsbuildinfo",
diff --git a/packages/codemirror/tsconfig.lib.json b/packages/codemirror/tsconfig.lib.json
index f3f86f56fc..53a8df3b12 100644
--- a/packages/codemirror/tsconfig.lib.json
+++ b/packages/codemirror/tsconfig.lib.json
@@ -1,7 +1,6 @@
 {
   "extends": "../../tsconfig.base.json",
   "compilerOptions": {
-    "baseUrl": ".",
     "rootDir": "src",
     "outDir": "dist",
     "tsBuildInfoFile": "dist/tsconfig.lib.tsbuildinfo",
diff --git a/packages/commons/tsconfig.lib.json b/packages/commons/tsconfig.lib.json
index 31ab54c998..87373b221c 100644
--- a/packages/commons/tsconfig.lib.json
+++ b/packages/commons/tsconfig.lib.json
@@ -1,7 +1,6 @@
 {
   "extends": "../../tsconfig.base.json",
   "compilerOptions": {
-    "baseUrl": ".",
     "rootDir": "src",
     "outDir": "dist",
     "tsBuildInfoFile": "dist/tsconfig.lib.tsbuildinfo",
diff --git a/packages/express-partial-content/tsconfig.lib.json b/packages/express-partial-content/tsconfig.lib.json
index 3872b6af80..a03311402f 100644
--- a/packages/express-partial-content/tsconfig.lib.json
+++ b/packages/express-partial-content/tsconfig.lib.json
@@ -1,7 +1,6 @@
 {
   "extends": "../../tsconfig.base.json",
   "compilerOptions": {
-    "baseUrl": ".",
     "rootDir": "src",
     "outDir": "dist",
     "tsBuildInfoFile": "dist/tsconfig.lib.tsbuildinfo",
diff --git a/packages/highlightjs/tsconfig.lib.json b/packages/highlightjs/tsconfig.lib.json
index f46b85d22d..da6c0f8f69 100644
--- a/packages/highlightjs/tsconfig.lib.json
+++ b/packages/highlightjs/tsconfig.lib.json
@@ -1,7 +1,6 @@
 {
   "extends": "../../tsconfig.base.json",
   "compilerOptions": {
-    "baseUrl": ".",
     "rootDir": "src",
     "outDir": "dist",
     "tsBuildInfoFile": "dist/tsconfig.lib.tsbuildinfo",
diff --git a/packages/share-theme/src/scripts/common/debounce.ts b/packages/share-theme/src/scripts/common/debounce.ts
index a4537d9631..bdebdad92c 100644
--- a/packages/share-theme/src/scripts/common/debounce.ts
+++ b/packages/share-theme/src/scripts/common/debounce.ts
@@ -1,5 +1,5 @@
 export default function debounce<T extends (...args: unknown[]) => unknown>(executor: T, delay: number) {
-    let timeout: NodeJS.Timeout | null;
+    let timeout: ReturnType<typeof setTimeout> | null;
     return function(...args: Parameters<T>): void {
         const callback = () => {
             timeout = null;
diff --git a/packages/turndown-plugin-gfm/tsconfig.lib.json b/packages/turndown-plugin-gfm/tsconfig.lib.json
index c4d01e6062..9c7bbb0d0f 100644
--- a/packages/turndown-plugin-gfm/tsconfig.lib.json
+++ b/packages/turndown-plugin-gfm/tsconfig.lib.json
@@ -1,7 +1,6 @@
 {
   "extends": "../../tsconfig.base.json",
   "compilerOptions": {
-    "baseUrl": ".",
     "rootDir": "src",
     "outDir": "dist",
     "tsBuildInfoFile": "dist/tsconfig.lib.tsbuildinfo",
diff --git a/tsconfig.base.json b/tsconfig.base.json
index 22906a79fc..0c1a4ef4ab 100644
--- a/tsconfig.base.json
+++ b/tsconfig.base.json
@@ -20,7 +20,7 @@
     "customConditions": ["development"],
     "verbatimModuleSyntax": false,  // TODO: Re-enable it when migrating back to ESM.
     "resolveJsonModule": true,
-    "downlevelIteration": true,
     "esModuleInterop": true,
+    "noUncheckedSideEffectImports": false,
   }
 }