HTML: test tools

HTML Markup Validation
validator.w3.org/#validate_by_uri+with_options

CSS Validation
jigsaw.w3.org/css-validator/#validate_by_uri+with_options

Broken Links Validator
validator.w3.org/checklink

webkoll dataskydd
webbkoll.dataskydd.net/de

Internationalization Checker
validator.w3.org/i18n-checker

SSL Labs
ssllabs.com/ssltest

GTmetrix
gtmetrix.com

Security Headers
securityheaders.com

Check Favicons
realfavicongenerator.net