- https://nibbler.insites.com/
- https://www.qualidator.com
- https://seitenreport.de/en
- https://try.powermapper.com/Demo/SortSite
- https://sitecheck.sucuri.net/
- https://securityheaders.com/
- https://observatory.mozilla.org/
- https://www.jitbit.com/sslcheck/
- https://mxtoolbox.com/blacklists.aspx
- https://mxtoolbox.com/SuperTool.aspx
- https://whois.domaintools.com/
- https://toolbox.googleapps.com/apps/dig/
- https://www.whatsmydns.net/
- https://gtmetrix.com/
- https://pagespeed.web.dev/
- https://tools.pingdom.com/
- https://www.webpagetest.org/
- http://www.gidnetwork.com/tools/gzip-test.php or https://www.whatsmyip.org/http-compression-test/ or https://checkgzipcompression.net/
- https://yellowlab.tools/
- https://tools.keycdn.com/http2-test
- https://tinypng.com/
- https://compresspng.com/
- https://www.jpegmini.com/ (App)
- https://imageoptim.com/mac (App)
- https://polypane.app/color-contrast/
- https://ffoodd.github.io/a11y.css/
- https://khan.github.io/tota11y/
- https://wave.webaim.org/
- https://fae.disability.illinois.edu/anonymous/
- https://validator.w3.org/checklink
- https://search.google.com/test/rich-results
- https://validator.schema.org/
- https://search.google.com/test/amp
- https://www.seoptimer.com/free-tools (e.g. https://www.seoptimer.com/alt-tag-checker)
- https://ahrefs.com/free-seo-tools
- https://www.onthemap.com/seo-browser/
- https://www.sistrix.de/kostenlose-tools/
- https://seominion.com/ (Browser Extension)
- https://developers.facebook.com/tools/debug/
- https://cards-dev.twitter.com/validator (Broken?)
- https://www.heymeta.com/
- https://www.linkedin.com/post-inspector/
- https://sicher3.de/google-fonts-checker/
- https://google-fonts-checker.54gradsoftware.de/de-DE
- https://www.ccm19.de/google-fonts-checker/
- https://www.deque.com/axe/devtools/ or https://www.deque.com/axe/browser-extensions/
- https://wave.webaim.org/extension/
- https://addons.mozilla.org/de/firefox/addon/google-lighthouse/
- Meta Pixel Helper (https://chrome.google.com/webstore/detail/meta-pixel-helper/fdgfkebogiimcoedlicjlajpkdmockpc)
- Tag Assistant for Conversions Beta (https://chrome.google.com/webstore/detail/tag-assistant-for-convers/llpfnmnallbompdmklfkcibfpcfpncdd)
Please add a comment for more checking tools, broken links or discontinued services. Thank you!
in the context of responsive images the linter for responsive images is quite useful.
the link to the repo: https://github.com/ausi/respimagelint
but it is used as a bookmarklet: https://ausi.github.io/respimagelint/
and in the context of a11y: https://accessibilityinsights.io (the automated tests are based on axe core but aside that it provides a set of manual tests to check for in addition)