HTML Markup Validation
HTML Tidy Browser Extension - Chrome Web Store
Also based on HTML Tidy
Simplest: Unclosed Tag Finder
Div Checker | www.tormus.com
Is is also very simple and nice.
Web developer has it.
Best: Validity - Chrome Web Store
Shit: HTML Validator - Chrome Web Store
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
The W3C Markup Validation Service