Can I Use
https://caniuse.com/

“Can I use” provides up-to-date browser support tables for support of front-end web technologies on desktop and mobile web browsers.

Browser Compatibility Data (BCD Watch)
https://bcd-watch.igalia.com/weekly/index.html

Web features & Baseline data
https://web-platform-dx.github.io/web-features/baseline-in-the-wild/

Can I Webview (The WebView Compatibility Data Project)
https://caniwebview.com/about/

Mobile Browser Market Share
https://gs.statcounter.com/browser-market-share/mobile/united-states-of-america

Cross Browser Compatibility Issues to Avoid
https://www.browserstack.com/guide/common-cross-browser-compatibility-issues

Usage share of web browsers
https://en.wikipedia.org/wiki/Usage_share_of_web_browsers

Screen Readers and Browsers!
Which is the Best Combination for Accessibility Testing?
https://www.digitala11y.com/screen-readers-browsers-which-is-the-best-combination-for-accessibility-testing/

Why one site can behave differently on different PCs and browsers.
https://blog.freedomscientific.com/responsive-web-design-why-one-site-can-behave-differently-on-different-pcs-and-browsers/