Guide · Agent skill
How to use the HTML5 Doctype Checker agent skill
Updated · written with AI from the project's own documentation and checked against our catalogue data
The doctype skill teaches an AI agent to check whether an HTML page begins with the modern HTML5 doctype, <!DOCTYPE html>. This guide explains what the skill does, how to install it from a ZIP file, how to ask for a check in plain language, and what to watch for. You do not need prior experience with agent skills to follow it.
HTML5 Doctype Checkerthedaviddias/Front-End-Checklist · MIT Download the skill Quick start
- 1Download the ZIP. Get the doctype skill ZIP file from this skill's page in the directory.
- 2Unzip into a skills folder. Extract it into ~/.claude/skills/ for personal use, or into your project's .claude/skills/ folder.
- 3Or upload it in the Claude apps. Go to Settings → Capabilities → Skills and upload the ZIP.
- 4Ask for a doctype review. Request a check of an HTML page or template in plain language, such as asking whether its doctype is correct.
- 5Give the agent the source. Provide the template file or the raw page source so the agent can read the first line.
- 6Apply the fix and verify. Add <!DOCTYPE html> as the first line, remove old doctypes, then confirm in the rendered HTML.
What the doctype skill makes the agent do
The doctype declaration tells the browser which version of HTML a document uses. With <!DOCTYPE html>, browsers use standards rendering. Without it, browsers enter Quirks Mode, a legacy compatibility mode that changes CSS behaviour such as the box model and vertical centring.
When you ask the agent to review a page or template, this skill tells it to look at the very first line of the raw HTML source, not the rendered DOM. It flags missing, outdated or misplaced doctypes and suggests a fix. For server-rendered frameworks, it checks the root HTML template, such as _document.tsx in Next.js, index.html in Vite or Vue, or application.html.erb in Rails.
- Flags a missing doctype
- Flags old HTML4 or XHTML doctypes (those containing PUBLIC and a DTD URL)
- Flags whitespace, comments or a BOM before the doctype
- Suggests adding <!DOCTYPE html> as the first line, before <html>
Install the doctype skill
Download the ZIP file from the skill's page on this directory. Then choose one of the two options below.
For a personal install, unzip it into ~/.claude/skills/. For a single project, unzip it into that project's .claude/skills/ folder. The skill is made of two files: SKILL.md and references/rule.md.
Alternatively, upload the ZIP in the Claude apps under Settings → Capabilities → Skills.
- Personal: ~/.claude/skills/
- Project only: .claude/skills/ inside your project
- Claude apps: Settings → Capabilities → Skills, then upload the ZIP
Trigger the skill with natural requests
You do not need a special command. Ask the agent in plain language about HTML documents, pages or templates. The skill applies to all HTML documents, so requests that mention the doctype, Quirks Mode or a page's first line should match it.
Give the agent access to the actual files or the page source. The skill says the agent should check the raw HTML source, for example through view-source: or DevTools > Network > Response.
- Review this index.html and tell me whether the doctype is correct.
- Check the root template in our Next.js app for a missing doctype.
- Audit these legacy pages for old XHTML or HTML 4.01 doctypes.
- Why is my layout rendering differently in one browser? Check the doctype first.
Example: fixing a legacy page
Suppose a page starts with an XHTML 1.0 Transitional doctype or has no doctype at all. The skill's reference examples show the correct form: <!DOCTYPE html> on the first line, followed by <html lang="en">, a charset meta tag, a viewport meta tag and a title.
The agent should recommend replacing the old doctype or adding the new one as the absolute first line, then confirm the result in the rendered HTML by viewing the page source in the browser.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Page Title</title>
</head>
<body>
<!-- content -->
</body>
</html>Safety notes: scripts and network access
This skill runs no scripts and uses no network access, according to its listing. It works from the files and page source you provide. Its requirements list nothing beyond that.
The skill's limits matter too. It covers only the doctype rule, not full HTML validation. If the agent cannot reach the rendered page or the template files, it can only guess from partial information, so give it the files or the source.
- Runs scripts: no
- Uses network: no
- Requires: nothing listed
Tips for better results
Ask for the check on the raw source, and for framework projects, ask the agent to check both the root template and the rendered output. Frameworks can add the doctype for you, so the template and the output may differ.
Pair this skill with other checks when you need more than the doctype. It will not catch other HTML errors.
- Give the agent the template file and the rendered HTML together
- Keep the doctype uppercase as <!DOCTYPE html> for the conventional form
- Make sure nothing, not even whitespace, appears before the doctype
What people use it for
- Auditing a legacy site that still uses HTML 4.01 or XHTML doctypes
- Checking the root template of a Next.js, Vite or Rails project
- Diagnosing layout differences caused by Quirks Mode
- Teaching a beginner why the first line of an HTML file matters
Frequently asked questions
How do I install the doctype agent skill?
Download the ZIP from this skill's page and unzip it into ~/.claude/skills/ for personal use, or into a project's .claude/skills/ folder. You can also upload the ZIP in the Claude apps under Settings → Capabilities → Skills.
What does the doctype skill check?
It checks whether the first line of the raw HTML source is <!DOCTYPE html>. It flags missing doctypes, old HTML4 or XHTML doctypes, and anything placed before the doctype, then suggests a fix.
Does the doctype skill need any requirements?
The listing states that it requires nothing and runs no scripts. It does not use the network either.
Why does the doctype matter?
Without <!DOCTYPE html>, browsers can enter Quirks Mode, which changes CSS behaviour such as the box model and can cause inconsistent rendering across browsers.
Does this skill validate my whole HTML?
No. The skill covers only the doctype rule. It is not a deep HTML validator, so use other tools for broader checks.
What if the agent cannot see my template or page?
The skill's limits note that the agent can only guess from partial information when it cannot reach the rendered page or template files. Give it the files or the page source for a reliable check.
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Our verdict
Use this skill if you review sites, templates or legacy pages and want a focused check that the first line is the HTML5 doctype. It is a good beginner-friendly tool because it runs no scripts and uses no network. Pick a broader HTML validator if you need checks beyond the doctype.
Download the skill