Website Audit Tools

Accessibility

Color Safe
Tool to help designers create accessible color palettes
Google Mobile-Friendly Test
Tests a variety of factors for mobile accessibility
WAVE Web Accessibility Evaluation Tool
Check for ADA and WCAG compliance issues
WebAIM Contrast Checker
Compare any two colors to check their contrast for readability
W3C Internationalization Checker
Check for internationalization support
W3C Link Checker
Find broken links

Validators

Unicorn
W3C’s unified validator for HTML, CSS, RSS and more
W3C Spell Checker
Find misspelled words

SEO

Google Analytics
Extensive site usage reporting (requires proper site configuration)
Google Search Console
Formerly Google Webmaster Tools (requires site ownership validation)
Neil Patel SEO Analyzer
A simple, general-purpose SEO reporting tool

General Links

W3C Developer Tools
W3C’s full suite of site checking tools