I'm not the best UI designer so I've compiled some browser-based tools I use on a regular basis to help me make or edit front-end stuff.
Play around with Google Fonts on your site! This tool allows you to change the font of any text node on the page you navigate to.
All in one validator! W3C's unified validator, which performs a variety of checks from the popular HTML and CSS validators, as well as other useful services.
Edit this Page
Clicking this link will enable designMode on your browser and allow you to edit text content on the page you are on (in this case, this page). That changes are all local though :) You might want to drag this to your bookmarks.
The best color palette generator I've used so far.
An advanced eyedropper tool (with a bunch of other useful functionality) in the convenient form of a chrome extension.