I need to check my HTML or CSS for errors
Broken layout or flaky rendering often traces to invalid markup. Validate first and fix the exact lines.
To catch problems before they ship, validate your HTML or CSS: a validator flags unclosed tags, invalid nesting, unknown properties and typos that break rendering or SEO. Our HTML and CSS Validators check your code in your browser, so nothing is uploaded. Fix the flagged lines and your page renders reliably across browsers.
4 tools for this
Each one is free, private and runs in your browser. No sign-up, no watermark.
HTML Validator
Paste HTML and instantly check well-formedness plus on-page SEO (title, meta description, lang, image alt text and heading order) with a live outline. 100% in your browser.
CSS Validator
Paste your CSS and instantly catch unbalanced braces, unclosed and empty rules, and unterminated comments or strings, each pinpointed to a line, 100% in your browser.
Meta Tag Analyzer
Paste a page's HTML to audit its title, description, canonical, robots, Open Graph and Twitter Card tags: with a length score, missing/duplicate checks and a live search & social preview, 100% in your browser.
Heading Checker
Paste your HTML to see the full H1–H6 outline as a tree and catch structural problems (missing H1, multiple H1s, skipped levels and empty headings) with a plain-English fix for each. 100% in your browser.
How to fix it
Follow these steps and you'll have it sorted in a couple of minutes.
- 1
Open the validator
HTML → HTML Validator, styles → CSS Validator. Paste your code in: it's checked in your browser.
- 2
Review the errors
It flags unclosed tags, bad nesting, unknown properties and typos, with the line for each.
- 3
Fix the flagged lines
Work through the issues. Invalid markup is a common cause of cross-browser rendering bugs.
- 4
Re-check and ship
Validate again until it's clean, then check headings and meta with the SEO tools.
- Ignoring warnings that seem minor. An unclosed tag can break the whole layout below it.
- Validating the wrong thing (running CSS through the HTML checker). Match the tool to the code.
- Shipping without checking headings/meta too, which affects SEO as well as rendering.
Frequently asked questions
More problems we solve
Ran into one of these too? Here's the fix.
- I want to check my page's SEO and meta tagsSee your page the way Google and social platforms do, then fix the missing title, description or heading structure.
- My site has no meta tags or preview when sharedA blank preview costs you clicks. Add the right tags so Google and social show a proper title, description and image.
- My code or data is minified and unreadableOne long line of minified everything? Pick the matching formatter and get properly indented, readable code back.
Keep exploring
Problems we solve
Definitions
From the blog
Compare formats
By file type
Ready? Start with HTML Validator
Free, private and instant: everything runs in your browser.