This is an incomplete collection of frontend testing tips, tricks and tools. It's just the stuff I've found useful over the years. Feel free to suggest more stuff. I'll add it if I like it ;)
- MacOS: Xcode Device Simulator: https://unrealnavigation.com/blog/web-inspector-with-ios-simulator
- Inspector for real iOS devices: https://webdesign.tutsplus.com/articles/quick-tip-using-web-inspector-to-debug-mobile-safari--webdesign-8787
- Inspector for real Android devices: https://developers.google.com/web/tools/chrome-devtools/remote-debugging/
- https://www.browserstack.com/
- Chrome inspector > network tab (disable cache; "good 3G" restricted pageload < 5s; transferred data < 1MB; requests < 20)
- https://developers.google.com/speed/pagespeed/insights/ (values > 80; ignore "above the fold" stuff - it's probably not handy/useful for large pages)
- https://developers.google.com/web/tools/lighthouse/run
- https://testmysite.withgoogle.com (same data as pagespeed with nicer visuals; might be usefult to show to a customer)
- https://gtmetrix.com/
- https://www.webpagetest.org/ (might have long queues)
- https://tools.pingdom.com/
- https://developers.google.com/web/tools/lighthouse/ testing lots of stuff
- https://pageweight.imgix.com/
- https://webspeedtest.cloudinary.com/ tests a lot of image stuff (when to prefer png over svg etc..)
- https://www.dotcom-tools.com/website-speed-test.aspx
- https://web.dev/measure
- https://treo.sh/sitespeed Chrome UX report data
- https://www.fastorslow.com/ Lighthouse based speed audit from multiple locations all over the world
- HTML: https://validator.w3.org/nu/ or https://html5.validator.nu
- CSS: https://jigsaw.w3.org/css-validator/ (don't take this too seriously as this might not validate some hacky selectors for specific browsers)
- Checklist: https://github.com/thedaviddias/Front-End-Checklist
- Does lots of validation & best practices stuff: https://sonarwhal.com/scanner/
- SSL: https://www.ssllabs.com/ssltest/ (aim for A/A+)
- SSL(advanced stuff): https://tls.imirhil.fr/
- https://securityheaders.io/
- CSP Analyzer: https://report-uri.io/home/analyse
- https://observatory.mozilla.org/
- https://www.htbridge.com/ssl/