Validators
HTML Validator.ru: Validator.nu
HTML W3C validator: https://validator.w3.org/#validate_by_upload+with_options
CSS W3C validator: https://jigsaw.w3.org/css-validator/#validate-by-input
CSSLint: http://csslint.net/
CSS Stylelint (VSCode): https://stylelint.io/
CSS Properties sort (PostCSS sort): https://dev.to/josebmendozam/automate-sorting-declarations-in-just-4-steps-49p4
Purge CSS: https://purgecss.com/CLI.html#usage
JavaScript ESLint (VSCode): https://marketplace.visualstudio.com/items?itemName=dbaeumer.vscode-eslint
Browser compatibility
Quirksmode: https://www.quirksmode.org/
Caniuse: https://caniuse.com/
CSS browser suport
Grid: https://gridbyexample.com/browsers/
CSS browser compatibility generators (prefix-ers)
Autoprefixer: https://autoprefixer.github.io/