fix: add remark plugin to render raw HTML as literal text (#16505)

* fix: add remark plugin to render raw HTML as literal text

Implemented a missing MDAST stage to neutralize raw HTML like major LLM WebUIs
do ensuring consistent and safe Markdown rendering

Introduced 'remarkLiteralHtml', a plugin that converts raw HTML nodes in the
Markdown AST into plain-text equivalents while preserving indentation and
line breaks. This ensures consistent rendering and prevents unintended HTML
execution, without altering valid Markdown structure

Kept 'remarkRehype' in the pipeline since it performs the required conversion
from MDAST to HAST for KaTeX, syntax highlighting, and HTML serialization

Refined the link-enhancement logic to skip unnecessary DOM rewrites,
fixing a subtle bug where extra paragraphs were injected after the first
line due to full innerHTML reconstruction, and ensuring links open in new
tabs only when required

Final pipeline: remarkGfm -> remarkMath -> remarkBreaks -> remarkLiteralHtml
-> remarkRehype -> rehypeKatex -> rehypeHighlight -> rehypeStringify

* fix: address review feedback from allozaur

* chore: update webui build output

tools/server/webui/package-lock.json

@@ -50,6 +50,7 @@
 				"eslint-plugin-svelte": "^3.0.0",
 				"fflate": "^0.8.2",
 				"globals": "^16.0.0",
+				"mdast": "^3.0.0",
 				"mdsvex": "^0.12.3",
 				"playwright": "^1.53.0",
 				"prettier": "^3.4.2",
@@ -66,6 +67,7 @@
 				"tw-animate-css": "^1.3.5",
 				"typescript": "^5.0.0",
 				"typescript-eslint": "^8.20.0",
+				"unified": "^11.0.5",
 				"uuid": "^13.0.0",
 				"vite": "^7.0.4",
 				"vite-plugin-devtools-json": "^0.2.0",
@@ -2128,6 +2130,66 @@
 				"node": ">=14.0.0"
 			}
 		},
+		"node_modules/@tailwindcss/oxide-wasm32-wasi/node_modules/@emnapi/core": {
+			"version": "1.4.3",
+			"dev": true,
+			"inBundle": true,
+			"license": "MIT",
+			"optional": true,
+			"dependencies": {
+				"@emnapi/wasi-threads": "1.0.2",
+				"tslib": "^2.4.0"
+			}
+		},
+		"node_modules/@tailwindcss/oxide-wasm32-wasi/node_modules/@emnapi/runtime": {
+			"version": "1.4.3",
+			"dev": true,
+			"inBundle": true,
+			"license": "MIT",
+			"optional": true,
+			"dependencies": {
+				"tslib": "^2.4.0"
+			}
+		},
+		"node_modules/@tailwindcss/oxide-wasm32-wasi/node_modules/@emnapi/wasi-threads": {
+			"version": "1.0.2",
+			"dev": true,
+			"inBundle": true,
+			"license": "MIT",
+			"optional": true,
+			"dependencies": {
+				"tslib": "^2.4.0"
+			}
+		},
+		"node_modules/@tailwindcss/oxide-wasm32-wasi/node_modules/@napi-rs/wasm-runtime": {
+			"version": "0.2.11",
+			"dev": true,
+			"inBundle": true,
+			"license": "MIT",
+			"optional": true,
+			"dependencies": {
+				"@emnapi/core": "^1.4.3",
+				"@emnapi/runtime": "^1.4.3",
+				"@tybys/wasm-util": "^0.9.0"
+			}
+		},
+		"node_modules/@tailwindcss/oxide-wasm32-wasi/node_modules/@tybys/wasm-util": {
+			"version": "0.9.0",
+			"dev": true,
+			"inBundle": true,
+			"license": "MIT",
+			"optional": true,
+			"dependencies": {
+				"tslib": "^2.4.0"
+			}
+		},
+		"node_modules/@tailwindcss/oxide-wasm32-wasi/node_modules/tslib": {
+			"version": "2.8.0",
+			"dev": true,
+			"inBundle": true,
+			"license": "0BSD",
+			"optional": true
+		},
 		"node_modules/@tailwindcss/oxide-win32-arm64-msvc": {
 			"version": "4.1.11",
 			"resolved": "https://registry.npmjs.org/@tailwindcss/oxide-win32-arm64-msvc/-/oxide-win32-arm64-msvc-4.1.11.tgz",
@@ -4946,6 +5008,13 @@
 				"url": "https://github.com/sponsors/wooorm"
 			}
 		},
+		"node_modules/mdast": {
+			"version": "3.0.0",
+			"resolved": "https://registry.npmjs.org/mdast/-/mdast-3.0.0.tgz",
+			"integrity": "sha512-xySmf8g4fPKMeC07jXGz971EkLbWAJ83s4US2Tj9lEdnZ142UP5grN73H1Xd3HzrdbU5o9GYYP/y8F9ZSwLE9g==",
+			"dev": true,
+			"license": "MIT"
+		},
 		"node_modules/mdast-util-find-and-replace": {
 			"version": "3.0.2",
 			"resolved": "https://registry.npmjs.org/mdast-util-find-and-replace/-/mdast-util-find-and-replace-3.0.2.tgz",

tools/server/webui/package.json

@@ -52,6 +52,7 @@
 		"eslint-plugin-svelte": "^3.0.0",
 		"fflate": "^0.8.2",
 		"globals": "^16.0.0",
+		"mdast": "^3.0.0",
 		"mdsvex": "^0.12.3",
 		"playwright": "^1.53.0",
 		"prettier": "^3.4.2",
@@ -68,6 +69,7 @@
 		"tw-animate-css": "^1.3.5",
 		"typescript": "^5.0.0",
 		"typescript-eslint": "^8.20.0",
+		"unified": "^11.0.5",
 		"uuid": "^13.0.0",
 		"vite": "^7.0.4",
 		"vite-plugin-devtools-json": "^0.2.0",

tools/server/webui/src/lib/components/app/misc/MarkdownContent.svelte

@@ -14,6 +14,7 @@
 	import githubDarkCss from 'highlight.js/styles/github-dark.css?inline';
 	import githubLightCss from 'highlight.js/styles/github.css?inline';
 	import { mode } from 'mode-watcher';
+	import { remarkLiteralHtml } from '$lib/markdown/literal-html';
 
 	interface Props {
 		content: string;
@@ -50,36 +51,59 @@
 			.use(remarkGfm) // GitHub Flavored Markdown
 			.use(remarkMath) // Parse $inline$ and $$block$$ math
 			.use(remarkBreaks) // Convert line breaks to <br>
-			.use(remarkRehype) // Convert to rehype (HTML AST)
+			.use(remarkLiteralHtml) // Treat raw HTML as literal text with preserved indentation
+			.use(remarkRehype) // Convert Markdown AST to rehype
 			.use(rehypeKatex) // Render math using KaTeX
 			.use(rehypeHighlight) // Add syntax highlighting
 			.use(rehypeStringify); // Convert to HTML string
 	});
 
 	function enhanceLinks(html: string): string {
+		if (!html.includes('<a')) {
+			return html;
+		}
+
 		const tempDiv = document.createElement('div');
 		tempDiv.innerHTML = html;
 
 		// Make all links open in new tabs
 		const linkElements = tempDiv.querySelectorAll('a[href]');
+		let mutated = false;
+
 		for (const link of linkElements) {
+			const target = link.getAttribute('target');
+			const rel = link.getAttribute('rel');
+
+			if (target !== '_blank' || rel !== 'noopener noreferrer') {
+				mutated = true;
+			}
+
 			link.setAttribute('target', '_blank');
 			link.setAttribute('rel', 'noopener noreferrer');
 		}
 
-		return tempDiv.innerHTML;
+		return mutated ? tempDiv.innerHTML : html;
 	}
 
 	function enhanceCodeBlocks(html: string): string {
+		if (!html.includes('<pre')) {
+			return html;
+		}
+
 		const tempDiv = document.createElement('div');
 		tempDiv.innerHTML = html;
 
 		const preElements = tempDiv.querySelectorAll('pre');
+		let mutated = false;
 
 		for (const [index, pre] of Array.from(preElements).entries()) {
 			const codeElement = pre.querySelector('code');
 
-			if (!codeElement) continue;
+			if (!codeElement) {
+				continue;
+			}
+
+			mutated = true;
 
 			let language = 'text';
 			const classList = Array.from(codeElement.classList);
@@ -127,7 +151,7 @@
 			pre.parentNode?.replaceChild(wrapper, pre);
 		}
 
-		return tempDiv.innerHTML;
+		return mutated ? tempDiv.innerHTML : html;
 	}
 
 	async function processMarkdown(text: string): Promise<string> {

tools/server/webui/src/lib/constants/literal-html.ts

@@ -0,0 +1,15 @@
+export const LINE_BREAK = /\r?\n/;
+
+export const PHRASE_PARENTS = new Set([
+	'paragraph',
+	'heading',
+	'emphasis',
+	'strong',
+	'delete',
+	'link',
+	'linkReference',
+	'tableCell'
+]);
+
+export const NBSP = '\u00a0';
+export const TAB_AS_SPACES = NBSP.repeat(4);

tools/server/webui/src/lib/markdown/literal-html.ts

@@ -0,0 +1,121 @@
+import type { Plugin } from 'unified';
+import { visit } from 'unist-util-visit';
+import type { Break, Content, Paragraph, PhrasingContent, Root, Text } from 'mdast';
+import { LINE_BREAK, NBSP, PHRASE_PARENTS, TAB_AS_SPACES } from '$lib/constants/literal-html';
+
+/**
+ * remark plugin that rewrites raw HTML nodes into plain-text equivalents.
+ *
+ * remark parses inline HTML into `html` nodes even when we do not want to render
+ * them. We turn each of those nodes into regular text (plus `<br>` break markers)
+ * so the downstream rehype pipeline escapes the characters instead of executing
+ * them. Leading spaces and tab characters are converted to non‑breaking spaces to
+ * keep indentation identical to the original author input.
+ */
+
+function preserveIndent(line: string): string {
+	let index = 0;
+	let output = '';
+
+	while (index < line.length) {
+		const char = line[index];
+
+		if (char === ' ') {
+			output += NBSP;
+			index += 1;
+			continue;
+		}
+
+		if (char === '\t') {
+			output += TAB_AS_SPACES;
+			index += 1;
+			continue;
+		}
+
+		break;
+	}
+
+	return output + line.slice(index);
+}
+
+function createLiteralChildren(value: string): PhrasingContent[] {
+	const lines = value.split(LINE_BREAK);
+	const nodes: PhrasingContent[] = [];
+
+	for (const [lineIndex, rawLine] of lines.entries()) {
+		if (lineIndex > 0) {
+			nodes.push({ type: 'break' } as Break as unknown as PhrasingContent);
+		}
+
+		nodes.push({
+			type: 'text',
+			value: preserveIndent(rawLine)
+		} as Text as unknown as PhrasingContent);
+	}
+
+	if (!nodes.length) {
+		nodes.push({ type: 'text', value: '' } as Text as unknown as PhrasingContent);
+	}
+
+	return nodes;
+}
+
+export const remarkLiteralHtml: Plugin<[], Root> = () => {
+	return (tree) => {
+		visit(tree, 'html', (node, index, parent) => {
+			if (!parent || typeof index !== 'number') {
+				return;
+			}
+
+			const replacement = createLiteralChildren(node.value);
+
+			if (!PHRASE_PARENTS.has(parent.type as string)) {
+				const paragraph: Paragraph = {
+					type: 'paragraph',
+					children: replacement as Paragraph['children'],
+					data: { literalHtml: true }
+				};
+
+				const siblings = parent.children as unknown as Content[];
+				siblings.splice(index, 1, paragraph as unknown as Content);
+
+				if (index > 0) {
+					const previous = siblings[index - 1] as Paragraph | undefined;
+
+					if (
+						previous?.type === 'paragraph' &&
+						(previous.data as { literalHtml?: boolean } | undefined)?.literalHtml
+					) {
+						const prevChildren = previous.children as unknown as PhrasingContent[];
+
+						if (prevChildren.length) {
+							const lastChild = prevChildren[prevChildren.length - 1];
+
+							if (lastChild.type !== 'break') {
+								prevChildren.push({
+									type: 'break'
+								} as Break as unknown as PhrasingContent);
+							}
+						}
+
+						prevChildren.push(...(paragraph.children as unknown as PhrasingContent[]));
+
+						siblings.splice(index, 1);
+
+						return index;
+					}
+				}
+
+				return index + 1;
+			}
+
+			(parent.children as unknown as PhrasingContent[]).splice(
+				index,
+				1,
+				...(replacement as unknown as PhrasingContent[])
+			);
+
+			return index + replacement.length;
+		});
+	};
+};