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/

Browser support (CSS, HTML, JavaScript)

Modernizr