HTML and CSS

The building blocks of the web

HTML

W3C Markup validator - Check that the markup (i.e. HTML) of your page is valid, syntactically correct and widely compatible.
HTML Semantics cheat sheet - A useful list of semantic HTML elements and descriptions of their uses.

CSS

W3C CCS Validator - Check that your CSS is valid, syntactically correct and widely compatible.

abstract illustration of three people
building a website by placing floating elements