HTML Markup Validation
Offline (inline)
HTML Tidy Browser Extension http://www.htmlkit.com/blog/html-tidy-browser-extension/ Works localy.
Also based on HTML Tidy
- Not maintained: Kingsquare HTML Validator https://www.kingsquare.nl/html-validator Not working and maintained
Online?
Simplest: Unclosed Tag Finder
Div Checker | www.tormus.com Is is also very simple and nice.
Web developer has it. Best: Validity Shit: HTML Validator
Check HTML markup
Validated | allan23/validated will put one simple button in a row of a page/post to check for W3C validation - simple and not epoch-changing.
Also, in Debug Objects plugin, now defunct, there was nice HTML Inspector option based on JS code from this project: philipwalton/html-inspector