Css Tricks http://css-tricks.com/
Tips, tricks, insights. Go here on a regular basis to learn and keep up.

Css Wizardry http://csswizardry.com/
Not as many posts recently, but very thoughtful and smart posts over the past couple of years. Worth reading through.

Filament Group Lab http://filamentgroup.com/lab/
Called ‘lab’ but mainly a blog or at least place with lots of great articles. These guys are great.

A List Apart http://www.alistapart.com
Venerable and still going strong.

Impressive Webs http://www.impressivewebs.com/


Mobify Blog http://www.mobify.com/blog/
Responsive web design posts, how-to pieces, thoughts

HTML5 Doctor http://html5doctor.com/

Position Absolute http://www.position-absolute.com/

Webkitbits http://www.webkitbits.com/
Blog about the engine behind Safari, Chrome, etc.