diff --git a/bun.lock b/bun.lock
index 379cf361b2..c6eb53154d 100644
--- a/bun.lock
+++ b/bun.lock
@@ -169,6 +169,7 @@
       "version": "0.7.45",
       "dependencies": {
         "@hyperframes/parsers": "workspace:*",
+        "htmlparser2": "^10.1.0",
         "linkedom": "^0.18.12",
         "postcss": "^8.5.8",
       },
diff --git a/packages/lint/package.json b/packages/lint/package.json
index 10f2d52838..58b3456fa5 100644
--- a/packages/lint/package.json
+++ b/packages/lint/package.json
@@ -54,6 +54,7 @@
   },
   "dependencies": {
     "@hyperframes/parsers": "workspace:*",
+    "htmlparser2": "^10.1.0",
     "linkedom": "^0.18.12",
     "postcss": "^8.5.8"
   },
diff --git a/packages/lint/src/context.ts b/packages/lint/src/context.ts
index 3e71b29a32..9638f3ac52 100644
--- a/packages/lint/src/context.ts
+++ b/packages/lint/src/context.ts
@@ -1,13 +1,10 @@
 import type { HyperframeLintFinding, HyperframeLinterOptions } from "./types";
 import {
-  extractBlocks,
-  extractOpenTags,
+  parseHtmlStructure,
   findRootTag,
   collectCompositionIds,
   readAttr,
   stripHtmlComments,
-  STYLE_BLOCK_PATTERN,
-  SCRIPT_BLOCK_PATTERN,
 } from "./utils";
 import type { OpenTag, ExtractedBlock } from "./utils";
 
@@ -34,19 +31,31 @@ export function buildLintContext(html: string, options: HyperframeLinterOptions
   // hijack the boundary match below. Linear + fixpoint (see stripHtmlComments) to
   // stay ReDoS-free and catch markers that re-form when a comment is removed.
   let source = stripHtmlComments(rawSource);
-  const sourceWithoutTemplates = source.replace(
-    /<template\b[^>]*>[\s\S]*?<\/template(?:\s[^>]*)?>/gi,
-    " ",
+  const initialStructure = parseHtmlStructure(source);
+  const templateTags = initialStructure.tags.filter(
+    (tag) => tag.name === "template" && tag.closeIndex != null,
   );
-  const templateMatch = source.match(/<template[^>]*>([\s\S]*)<\/template>/i);
+  let sourceWithoutTemplates = source;
+  for (const template of [...templateTags].reverse()) {
+    const end = template.endIndex ?? template.index;
+    sourceWithoutTemplates =
+      sourceWithoutTemplates.slice(0, template.index) +
+      " ".repeat(end - template.index) +
+      sourceWithoutTemplates.slice(end);
+  }
   // Some sub-composition files are HTML shells whose real root lives inside a
   // <template>. Keep nested templates intact when the visible document already
   // has a composition root; only unwrap when no root exists outside templates.
-  if (templateMatch?.[1] && !findRootTag(sourceWithoutTemplates)) source = templateMatch[1];
+  const template = templateTags[0];
+  let structure = initialStructure;
+  if (template && !findRootTag(sourceWithoutTemplates)) {
+    source = source.slice(template.index + template.raw.length, template.closeIndex);
+    structure = parseHtmlStructure(source);
+  }
 
-  const tags = extractOpenTags(source);
+  const tags = structure.tags;
   const styles = [
-    ...extractBlocks(source, STYLE_BLOCK_PATTERN),
+    ...structure.styles,
     ...(options.externalStyles ?? []).map((style) => ({
       attrs: `href="${style.href}"`,
       content: style.content,
@@ -54,9 +63,9 @@ export function buildLintContext(html: string, options: HyperframeLinterOptions
       index: -1,
     })),
   ];
-  const scripts = extractBlocks(source, SCRIPT_BLOCK_PATTERN);
+  const scripts = structure.scripts;
   const compositionIds = collectCompositionIds(tags);
-  const rootTag = findRootTag(source);
+  const rootTag = findRootTag(source, tags);
   const rootCompositionId = readAttr(rootTag?.raw || "", "data-composition-id");
 
   return {
diff --git a/packages/lint/src/hyperframeLinter.test.ts b/packages/lint/src/hyperframeLinter.test.ts
index 8590bc41c5..bc100f0d77 100644
--- a/packages/lint/src/hyperframeLinter.test.ts
+++ b/packages/lint/src/hyperframeLinter.test.ts
@@ -1,5 +1,7 @@
-import { describe, it, expect } from "vitest";
-import { lintHyperframeHtml } from "./hyperframeLinter.js";
+import { afterEach, describe, it, expect, vi } from "vitest";
+import { lintHyperframeHtml, lintMediaUrls } from "./hyperframeLinter.js";
+
+afterEach(() => vi.unstubAllGlobals());
 
 describe("lintHyperframeHtml — orchestrator", () => {
   const validComposition = `
@@ -122,3 +124,26 @@ describe("lintHyperframeHtml — orchestrator", () => {
     expect(rootFindings).toHaveLength(0);
   });
 });
+
+describe("lintMediaUrls", () => {
+  it("checks top-level remote media elements", async () => {
+    const fetchMock = vi.fn().mockResolvedValue({ ok: true, status: 200 });
+    vi.stubGlobal("fetch", fetchMock);
+
+    await lintMediaUrls('<img id="hero" src="https://example.com/hero.png">');
+
+    expect(fetchMock).toHaveBeenCalledWith(
+      "https://example.com/hero.png",
+      expect.objectContaining({ method: "HEAD" }),
+    );
+  });
+
+  it("ignores remote media embedded inside an iframe srcdoc attribute", async () => {
+    const fetchMock = vi.fn();
+    vi.stubGlobal("fetch", fetchMock);
+
+    await lintMediaUrls(`<iframe srcdoc='<img src="https://example.com/embedded.png">'></iframe>`);
+
+    expect(fetchMock).not.toHaveBeenCalled();
+  });
+});
diff --git a/packages/lint/src/hyperframeLinter.ts b/packages/lint/src/hyperframeLinter.ts
index 1dc88067c6..b28e3f6ca8 100644
--- a/packages/lint/src/hyperframeLinter.ts
+++ b/packages/lint/src/hyperframeLinter.ts
@@ -1,6 +1,6 @@
 import type { HyperframeLintFinding, HyperframeLintResult, HyperframeLinterOptions } from "./types";
 import { buildLintContext } from "./context";
-import { readAttr, truncateSnippet } from "./utils";
+import { parseHtmlStructure, readAttr, truncateSnippet } from "./utils";
 import { coreRules } from "./rules/core";
 import { mediaRules } from "./rules/media";
 import { gsapRules } from "./rules/gsap";
@@ -73,11 +73,8 @@ function extractMediaUrls(html: string): Array<{
     elementId?: string;
     snippet: string;
   }> = [];
-  const tagRe = /<(video|audio|img|source)\b[^>]*>/gi;
-  let match: RegExpExecArray | null;
-  while ((match = tagRe.exec(html)) !== null) {
-    const tagName = (match[1] ?? "").toLowerCase();
-    const raw = match[0];
+  for (const { name: tagName, raw } of parseHtmlStructure(html).tags) {
+    if (!/^(?:video|audio|img|source)$/.test(tagName)) continue;
     const src = readAttr(raw, "src");
     if (!src) continue;
     if (/^https?:\/\//i.test(src)) {
diff --git a/packages/lint/src/project.test.ts b/packages/lint/src/project.test.ts
index 8fc82aa24b..a2705d4ced 100644
--- a/packages/lint/src/project.test.ts
+++ b/packages/lint/src/project.test.ts
@@ -209,3 +209,29 @@ describe("missing_or_empty_sub_composition", () => {
     expect(finding?.message).toContain("compositions/does-not-exist.html");
   });
 });
+
+describe("template shell style sources", () => {
+  it("collects links, style blocks, and inline styles from template content", async () => {
+    const project = makeProject(`<html><body>
+      <div id="scene" data-composition-id="main" data-width="1920" data-height="1080" data-start="0" data-duration="10"></div>
+      <template data-composition-id="shell">
+        <link rel="stylesheet" href="shell.css">
+        <style>[data-composition-id="main"] .title { opacity: 0; }</style>
+        <div style="mask-image: url(missing-inline-mask.png)"></div>
+        <template><style>[data-composition-id="main"] .nested { opacity: 0; }</style></template>
+      </template>
+      <script>window.__timelines = {};</script>
+    </body></html>`);
+    writeFileSync(
+      join(project, "shell.css"),
+      '[data-composition-id="main"] .from-link { opacity: 0; }',
+    );
+
+    const { results } = await lintProject(project);
+    const findings = results.flatMap((entry) => entry.result.findings);
+    expect(
+      findings.filter((finding) => finding.code === "composition_self_attribute_selector"),
+    ).toHaveLength(3);
+    expect(findings.some((finding) => finding.code === "texture_mask_asset_not_found")).toBe(true);
+  });
+});
diff --git a/packages/lint/src/project.ts b/packages/lint/src/project.ts
index 3419eed4a9..52d828c1db 100644
--- a/packages/lint/src/project.ts
+++ b/packages/lint/src/project.ts
@@ -24,6 +24,19 @@ interface CssSource {
   rootRelativePath?: string;
 }
 
+/** Linkedom keeps template contents in a DocumentFragment that is not part of
+ * the document query tree. Lint rules must still see shell styles and links
+ * inside templates, so walk each template's content recursively without
+ * falling back to regex parsing. */
+function querySelectorAllIncludingTemplates(root: ParentNode, selector: string): Element[] {
+  const matches: Element[] = [...root.querySelectorAll(selector)];
+  for (const template of root.querySelectorAll("template")) {
+    const content = (template as HTMLTemplateElement).content;
+    if (content) matches.push(...querySelectorAllIncludingTemplates(content, selector));
+  }
+  return matches;
+}
+
 export interface ProjectLintResult {
   results: Array<{ file: string; result: HyperframeLintResult }>;
   totalErrors: number;
@@ -32,75 +45,67 @@ export interface ProjectLintResult {
 }
 
 const AUDIO_EXTENSIONS = new Set([".mp3", ".wav", ".aac", ".ogg", ".m4a", ".flac", ".opus"]);
-const STYLE_BLOCK_RE = /<style\b[^>]*>([\s\S]*?)<\/style>/gi;
-const OPEN_TAG_RE = /<([a-z][\w:-]*)(\s[^<>]*?)?>/gi;
 const MASK_IMAGE_URL_RE =
   /\b(?:-webkit-)?mask-image\s*:\s*[^;{}]*url\(\s*(?:"([^"]+)"|'([^']+)'|([^"')\s]+))\s*\)/gi;
 
-function readHtmlAttr(tag: string, name: string): string | null {
-  const escaped = name.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
-  const match = tag.match(new RegExp(`\\b${escaped}\\s*=\\s*(?:"([^"]*)"|'([^']*)')`, "i"));
-  return match?.[1] ?? match?.[2] ?? null;
-}
-
 function isLocalStylesheetHref(href: string): boolean {
   return !!href && !/^(https?:|data:|blob:|\/\/)/i.test(href);
 }
 
-function collectExternalStyles(
+function collectLocalStylesheets(
   projectDir: string,
-  html: string,
+  document: ParentNode,
   compSrcPath?: string,
-): Array<{ href: string; content: string }> {
-  const styles: Array<{ href: string; content: string }> = [];
-  const linkRe = /<link\b[^>]*>/gi;
-  let match: RegExpExecArray | null;
-  while ((match = linkRe.exec(html)) !== null) {
-    const tag = match[0];
-    const rel = tag.match(/\brel\s*=\s*["']([^"']+)["']/i)?.[1] ?? "";
+): Array<{ href: string; content: string; rootRelativePath: string }> {
+  const styles: Array<{ href: string; content: string; rootRelativePath: string }> = [];
+  for (const link of querySelectorAllIncludingTemplates(document, "link")) {
+    const rel = link.getAttribute("rel") ?? "";
     if (!rel.split(/\s+/).some((part) => part.toLowerCase() === "stylesheet")) continue;
-    const href = tag.match(/\bhref\s*=\s*["']([^"']+)["']/i)?.[1] ?? "";
+    const href = link.getAttribute("href") ?? "";
     if (!isLocalStylesheetHref(href)) continue;
     const rootRelative = compSrcPath ? join(dirname(compSrcPath), href) : href;
     const stylesheet = resolveExistingLocalAsset(projectDir, rootRelative);
     if (!stylesheet) continue;
-    styles.push({ href, content: readFileSync(stylesheet.resolved, "utf-8") });
+    styles.push({
+      href,
+      content: readFileSync(stylesheet.resolved, "utf-8"),
+      rootRelativePath: stylesheet.rootRelativePath,
+    });
+  }
+  return styles;
+}
+
+function collectExternalStyles(
+  projectDir: string,
+  html: string,
+  compSrcPath?: string,
+): Array<{ href: string; content: string }> {
+  const styles: Array<{ href: string; content: string }> = [];
+  const { document } = parseHTML(html);
+  for (const { href, content } of collectLocalStylesheets(projectDir, document, compSrcPath)) {
+    styles.push({ href, content });
   }
   return styles;
 }
 
 function collectCssSources(projectDir: string, html: string, compSrcPath?: string): CssSource[] {
   const sources: CssSource[] = [];
+  const { document } = parseHTML(html);
 
-  let styleMatch: RegExpExecArray | null;
-  const stylePattern = new RegExp(STYLE_BLOCK_RE.source, STYLE_BLOCK_RE.flags);
-  while ((styleMatch = stylePattern.exec(html)) !== null) {
-    sources.push({ content: styleMatch[1] ?? "" });
+  for (const style of querySelectorAllIncludingTemplates(document, "style")) {
+    sources.push({ content: style.textContent ?? "" });
   }
 
-  const linkRe = /<link\b[^>]*>/gi;
-  let linkMatch: RegExpExecArray | null;
-  while ((linkMatch = linkRe.exec(html)) !== null) {
-    const tag = linkMatch[0];
-    const rel = readHtmlAttr(tag, "rel") ?? "";
-    if (!rel.split(/\s+/).some((part) => part.toLowerCase() === "stylesheet")) continue;
-    const href = readHtmlAttr(tag, "href") ?? "";
-    if (!isLocalStylesheetHref(href)) continue;
-
-    const rootRelativePath = compSrcPath ? join(dirname(compSrcPath), href) : href;
-    const stylesheet = resolveExistingLocalAsset(projectDir, rootRelativePath);
-    if (!stylesheet) continue;
-    sources.push({
-      content: readFileSync(stylesheet.resolved, "utf-8"),
-      rootRelativePath: stylesheet.rootRelativePath,
-    });
+  for (const { content, rootRelativePath } of collectLocalStylesheets(
+    projectDir,
+    document,
+    compSrcPath,
+  )) {
+    sources.push({ content, rootRelativePath });
   }
 
-  let tagMatch: RegExpExecArray | null;
-  const tagPattern = new RegExp(OPEN_TAG_RE.source, OPEN_TAG_RE.flags);
-  while ((tagMatch = tagPattern.exec(html)) !== null) {
-    const tag = tagMatch[0];
-    const style = readHtmlAttr(tag, "style");
+  for (const element of querySelectorAllIncludingTemplates(document, "[style]")) {
+    const style = element.getAttribute("style");
     if (!style) continue;
     sources.push({ content: style });
   }
diff --git a/packages/lint/src/rules/composition.ts b/packages/lint/src/rules/composition.ts
index 706d0e5442..0a52be380f 100644
--- a/packages/lint/src/rules/composition.ts
+++ b/packages/lint/src/rules/composition.ts
@@ -1,4 +1,4 @@
-import type { LintContext, HyperframeLintFinding, ExtractedBlock } from "../context";
+import type { LintContext, HyperframeLintFinding, ExtractedBlock, OpenTag } from "../context";
 import {
   findHtmlTag,
   readAttr,
@@ -132,12 +132,11 @@ function collectDeclaredVariableIds(htmlTagRaw: string): Set<string> | null {
  * template/fragment sub-comps hold it on their composition root div. Returns
  * null if any occurrence has unparseable JSON.
  */
-function collectAllDeclaredVariableIds(source: string): Set<string> | null {
+function collectAllDeclaredVariableIds(tags: readonly OpenTag[]): Set<string> | null {
   const all = new Set<string>();
-  const tagRe = /<[a-zA-Z][^>]*\bdata-composition-variables\b[^>]*>/gi;
-  let match: RegExpExecArray | null;
-  while ((match = tagRe.exec(source)) !== null) {
-    const ids = collectDeclaredVariableIds(match[0]);
+  for (const tag of tags) {
+    if (!readAttr(tag.raw, "data-composition-variables")) continue;
+    const ids = collectDeclaredVariableIds(tag.raw);
     if (ids === null) return null;
     for (const id of ids) all.add(id);
   }
@@ -150,10 +149,10 @@ function collectAllDeclaredVariableIds(source: string): Set<string> | null {
  * `<html>` and no declarations of its own (its values come from a host's
  * data-variable-values, which this file can't see).
  */
-function declaredIdsForBindingCheck(source: string): Set<string> | null {
-  const declared = collectAllDeclaredVariableIds(source);
+function declaredIdsForBindingCheck(tags: readonly OpenTag[]): Set<string> | null {
+  const declared = collectAllDeclaredVariableIds(tags);
   if (declared === null) return null;
-  if (declared.size === 0 && !findHtmlTag(source)) return null;
+  if (declared.size === 0 && !findHtmlTag(tags)) return null;
   return declared;
 }
 
@@ -653,11 +652,11 @@ export const compositionRules: Array<(ctx: LintContext) => HyperframeLintFinding
   // nothing, so a typo'd binding is invisible until a customer's override
   // does nothing. Skipped for fragment files (no <html>): their values come
   // from a host's data-variable-values, which this file can't see.
-  ({ source, tags }) => {
+  ({ tags }) => {
     // Declarations live on <html> (full-document comps) OR the composition root
     // div (template/fragment sub-comps); declaredIdsForBindingCheck unions both
     // and returns null for files this rule should skip.
-    const declared = declaredIdsForBindingCheck(source);
+    const declared = declaredIdsForBindingCheck(tags);
     if (!declared) return [];
     const findings: HyperframeLintFinding[] = [];
     for (const tag of tags) {
@@ -683,8 +682,8 @@ export const compositionRules: Array<(ctx: LintContext) => HyperframeLintFinding
   // catch them at lint time rather than wondering why their `getVariables()`
   // defaults aren't applied.
   // fallow-ignore-next-line complexity
-  ({ source }) => {
-    const htmlTag = findHtmlTag(source);
+  ({ tags }) => {
+    const htmlTag = findHtmlTag(tags);
     if (!htmlTag) return [];
     const raw = readJsonAttr(htmlTag.raw, "data-composition-variables");
     if (!raw) return [];
@@ -763,8 +762,8 @@ export const compositionRules: Array<(ctx: LintContext) => HyperframeLintFinding
   // fixed top-left-origin screenshot region, which RTL layout can shift the
   // actual content away from), only surfaces the already-confirmed footgun
   // before someone hits it blind.
-  ({ source }) => {
-    const htmlTag = findHtmlTag(source);
+  ({ tags }) => {
+    const htmlTag = findHtmlTag(tags);
     if (!htmlTag) return [];
     const dir = readAttr(htmlTag.raw, "dir");
     if (!dir) return [];
diff --git a/packages/lint/src/rules/core.test.ts b/packages/lint/src/rules/core.test.ts
index f3c0014350..cf5e463e49 100644
--- a/packages/lint/src/rules/core.test.ts
+++ b/packages/lint/src/rules/core.test.ts
@@ -77,6 +77,46 @@ ${headContent}
 }
 
 describe("core rules", () => {
+  it("does not lint scripts embedded inside an iframe srcdoc attribute", async () => {
+    const html = `
+<html><body>
+  <div data-composition-id="root" data-width="1280" data-height="720"></div>
+  <iframe srcdoc="<script>const child = gsap.timeline({ paused: true }); child.to(&quot;#x&quot;, { opacity: 1 });</script>"></iframe>
+  <script src="https://cdn.jsdelivr.net/npm/gsap@3/dist/gsap.min.js"></script>
+  <script>
+    window.__timelines = window.__timelines || {};
+    const rootTl = gsap.timeline({ paused: true });
+    window.__timelines["root"] = rootTl;
+  </script>
+</body></html>`;
+
+    const result = await lintHyperframeHtml(html);
+
+    expect(
+      result.findings.find((finding) => finding.code === "invalid_inline_script_syntax"),
+    ).toBeUndefined();
+    expect(
+      result.findings.find((finding) => finding.code === "gsap_timeline_not_registered"),
+    ).toBeUndefined();
+  });
+
+  it("does not lint elements embedded inside an iframe srcdoc attribute", async () => {
+    const html = `
+<html><body>
+  <div data-composition-id="root" data-width="1280" data-height="720"></div>
+  <iframe srcdoc='<video src="child.mp4" data-start="0"></video>'></iframe>
+  <script>window.__timelines = {};</script>
+</body></html>`;
+
+    const result = await lintHyperframeHtml(html);
+
+    expect(
+      result.findings.find(
+        (finding) => finding.elementId === undefined && finding.message.includes("<video"),
+      ),
+    ).toBeUndefined();
+  });
+
   it("warns when an id starts with a digit and is unsafe in a hash selector", async () => {
     const html = `
 <html><body>
diff --git a/packages/lint/src/utils.ts b/packages/lint/src/utils.ts
index 4b2b2c7c4d..edb4b17e3a 100644
--- a/packages/lint/src/utils.ts
+++ b/packages/lint/src/utils.ts
@@ -1,11 +1,15 @@
 // Shared types, regex constants, and utility functions used across lint rule modules.
 // Nothing in this file should emit findings — it only parses and extracts.
 
+import { Parser } from "htmlparser2";
+
 export type OpenTag = {
   raw: string;
   name: string;
   attrs: string;
   index: number;
+  closeIndex?: number;
+  endIndex?: number;
 };
 
 export type ExtractedBlock = {
@@ -15,9 +19,6 @@ export type ExtractedBlock = {
   index: number;
 };
 
-const TAG_PATTERN = /<([a-z][\w:-]*)(\s[^<>]*?)?>/gi;
-export const STYLE_BLOCK_PATTERN = /<style\b([^>]*)>([\s\S]*?)<\/style>/gi;
-export const SCRIPT_BLOCK_PATTERN = /<script\b([^>]*)>([\s\S]*?)<\/script>/gi;
 const COMPOSITION_ID_IN_CSS_PATTERN = /\[data-composition-id=["']([^"']+)["']\]/g;
 export const TIMELINE_REGISTRY_INIT_PATTERN =
   /window\.__timelines\s*=\s*window\.__timelines\s*\|\|\s*\{\}|window\.__timelines\s*=\s*\{\}|window\.__timelines\s*\?\?=\s*\{\}/i;
@@ -51,36 +52,57 @@ const TIMELINE_REGISTRY_OBJECT_BODY_PATTERN = /window\.__timelines\s*=\s*\{([\s\
 const TIMELINE_REGISTRY_OBJECT_ENTRY_PATTERN =
   /(?:["']([^"']+)["']|([A-Za-z_$][\w$]*))\s*:\s*[A-Za-z_$][\w$]*/g;
 
-export function extractOpenTags(source: string): OpenTag[] {
+export function parseHtmlStructure(source: string): {
+  tags: OpenTag[];
+  scripts: ExtractedBlock[];
+  styles: ExtractedBlock[];
+} {
   const tags: OpenTag[] = [];
-  let match: RegExpExecArray | null;
-  const pattern = new RegExp(TAG_PATTERN.source, TAG_PATTERN.flags);
-  while ((match = pattern.exec(source)) !== null) {
-    const raw = match[0];
-    if (raw.startsWith("</") || raw.startsWith("<!")) continue;
-    tags.push({
-      raw,
-      name: (match[1] || "").toLowerCase(),
-      attrs: match[2] || "",
-      index: match.index,
-    });
-  }
-  return tags;
-}
+  const blocks = { script: [] as ExtractedBlock[], style: [] as ExtractedBlock[] };
+  const openTagsByName = new Map<string, OpenTag[]>();
+  const openBlocks: Array<{
+    name: "script" | "style";
+    attrs: string;
+    contentStart: number;
+    index: number;
+  }> = [];
+  const parser: Parser = new Parser(
+    {
+      onopentag(name) {
+        const index = parser.startIndex;
+        const raw = source.slice(index, parser.endIndex + 1);
+        const attrs = raw.slice(name.length + 1, -1).replace(/\s*\/$/, "");
+        const tag = { raw, name, attrs, index };
+        tags.push(tag);
+        const sameNameStack = openTagsByName.get(name) ?? [];
+        sameNameStack.push(tag);
+        openTagsByName.set(name, sameNameStack);
+        if (name === "script" || name === "style") {
+          openBlocks.push({ name, attrs, contentStart: parser.endIndex + 1, index });
+        }
+      },
+      onclosetag(name) {
+        const tag = openTagsByName.get(name)?.pop();
+        if (tag) {
+          tag.closeIndex = parser.startIndex;
+          tag.endIndex = parser.endIndex + 1;
+        }
+        if (name !== "script" && name !== "style") return;
+        const block = openBlocks.pop();
+        if (!block || block.name !== name) return;
+        blocks[name].push({
+          attrs: block.attrs,
+          content: source.slice(block.contentStart, parser.startIndex),
+          raw: source.slice(block.index, parser.endIndex + 1),
+          index: block.index,
+        });
+      },
+    },
+    { decodeEntities: false, lowerCaseAttributeNames: false, lowerCaseTags: true },
+  );
+  parser.end(source);
 
-export function extractBlocks(source: string, pattern: RegExp): ExtractedBlock[] {
-  const blocks: ExtractedBlock[] = [];
-  let match: RegExpExecArray | null;
-  const p = new RegExp(pattern.source, pattern.flags);
-  while ((match = p.exec(source)) !== null) {
-    blocks.push({
-      attrs: match[1] || "",
-      content: match[2] || "",
-      raw: match[0],
-      index: match.index,
-    });
-  }
-  return blocks;
+  return { tags, scripts: blocks.script, styles: blocks.style };
 }
 
 /**
@@ -89,41 +111,25 @@ export function extractBlocks(source: string, pattern: RegExp): ExtractedBlock[]
  * composition's visible root", whereas `<html>` is where document-level
  * metadata like `data-composition-variables` lives.
  */
-export function findHtmlTag(source: string): OpenTag | null {
-  const match = /<html\b([^<>]*)>/i.exec(source);
-  if (!match) return null;
-  return {
-    raw: match[0],
-    name: "html",
-    attrs: match[1] ?? "",
-    index: match.index,
-  };
+export function findHtmlTag(tags: readonly OpenTag[]): OpenTag | null {
+  return tags.find((tag) => tag.name === "html") ?? null;
 }
 
 // fallow-ignore-next-line complexity
-export function findRootTag(source: string): OpenTag | null {
-  const bodyOpenMatch = /<body\b([^>]*)>/i.exec(source);
-  const bodyCloseMatch = /<\/body>/i.exec(source);
+export function findRootTag(source: string, parsedTags?: readonly OpenTag[]): OpenTag | null {
+  const tags = parsedTags ?? parseHtmlStructure(source).tags;
+  const bodyTag = tags.find((tag) => tag.name === "body");
   if (
-    bodyOpenMatch &&
-    (readAttr(bodyOpenMatch[0], "data-composition-id") ||
-      readAttr(bodyOpenMatch[0], "data-width") ||
-      readAttr(bodyOpenMatch[0], "data-height"))
+    bodyTag &&
+    (readAttr(bodyTag.raw, "data-composition-id") ||
+      readAttr(bodyTag.raw, "data-width") ||
+      readAttr(bodyTag.raw, "data-height"))
   ) {
-    return {
-      raw: bodyOpenMatch[0],
-      name: "body",
-      attrs: bodyOpenMatch[1] ?? "",
-      index: bodyOpenMatch.index,
-    };
+    return bodyTag;
   }
-  const bodyStart = bodyOpenMatch ? bodyOpenMatch.index + bodyOpenMatch[0].length : 0;
-  const bodyEnd =
-    bodyOpenMatch && bodyCloseMatch && bodyCloseMatch.index > bodyStart
-      ? bodyCloseMatch.index
-      : source.length;
-  const bodyContent = bodyOpenMatch ? source.slice(bodyStart, bodyEnd) : source;
-  const bodyTags = extractOpenTags(bodyContent);
+  const bodyStart = bodyTag ? bodyTag.index + bodyTag.raw.length : 0;
+  const bodyEnd = bodyTag?.closeIndex ?? source.length;
+  const bodyTags = tags.filter((tag) => tag.index >= bodyStart && tag.index < bodyEnd);
   // Set when a leading <svg> defs block is skipped (see below) — extractOpenTags
   // is a flat, nesting-unaware scan, so without this the very next tag it
   // returns is the svg's own nested child (<defs>, <filter>, ...), not the
@@ -146,13 +152,12 @@ export function findRootTag(source: string): OpenTag | null {
       !readAttr(tag.raw, "data-width") &&
       !readAttr(tag.raw, "data-height")
     ) {
-      const closeMatch = /<\/svg\s*>/i.exec(bodyContent.slice(tag.index));
       // No closing tag found (malformed HTML) — skip everything rather than
       // risk returning one of the svg's own children as the root.
-      skipBefore = closeMatch ? tag.index + closeMatch.index + closeMatch[0].length : Infinity;
+      skipBefore = tag.endIndex ?? Infinity;
       continue;
     }
-    return { ...tag, index: tag.index + bodyStart };
+    return tag;
   }
   return null;
 }