Empfehlungen zum Testen von Webseiten:

HTML: https://validator.w3.org/

CSS: http://jigsaw.w3.org/css-validator/

Links: https://validator.w3.org/checklink

Favicons: https://realfavicongenerator.net/favicon_checker

Facebook debuggerg: https://developers.facebook.com/tools/debug/sharing/

HTML5 semantics test: https://gsnedders.html5.org/outliner/

page speed:

Webpagetest: https://www.webpagetest.org

Google: https://developers.google.com/speed/pagespeed/insights/