The WHATWG creates the HTML living standard, defining how browsers should implement HTML.
The W3 CSS working group creates the CSS specification, defining how browsers should implement CSS. The latest ready-for-production specification is Snapshot 2010.
Browsers come with stylesheets that load on every page: Firefox, Webkit (Safari), Chromium.
How Browsers Work How browser rendering engines work: HTML parsers, DOM and render tree construction, layout and painting.
Responsive images are coming to browsers soon with the <picture> element, srcset and sizes attribute. Picturefill is a polyfill so you can use this functionality in browsers that don't yet support them.
* { box-sizing: border-box } FTW
The Z-Index CSS Property: A Comprehensive Look
Create sprites with Instant Sprite
Breaking up with Sass and using PostCSS as a CSS pre-processor
CSS methodologies provide structural guidelines for organizing CSS. See OOCSS, SMACSS and BEM. Here's an everyman's description of all three.
Normalize.css makes browsers render all elements more consistently and in line with modern standards.
HTML5 Boilerplate, a lean HTML template of best-practices with plenty of documentation.
Can I use Browser support information for HTML/CSS properties/features.
CSS tricks A blog about CSS.