From Brackets' page:
One of the goals of Brackets is to make it easy to make quick edits to different bits of code without having to jump around between files.
Currently, Brackets has an early implementation of this. If you're in an HTML file, and you put the cursor inside a class or id attribute or a tag name, you can hit Ctrl/Cmd-E (for "edit"). Brackets will search the CSS files in the file tree for relevant rules, then open up an inline editor embedded in the HTML file that lets you make quick tweaks to one of the rules.
On the right side of the inline editor, Brackets shows the list of all rules that might be relevant to the selected tag/class/id. You can switch between the rules by clicking on them in the list, or using Alt-Up/Down Arrow.
You can make changes in the inline editor and save them, then close the editor by hitting Cmd-E again. Edits you make in the inline editor will be properly applied in Live Development mode (see below).
Some things to note about this feature:
Brackets doesn't take the cascade, tag context, etc. into account.
Brackets doesn't check to see which CSS files are linked into the current HTML file--it searches all files in the file tree.
Eventually Brackets could leverage inline editing for lots of other kinds of things--in addition to showing relevant JS code, you could imagine adding inline visual tools like gradient and shadow editors for the selected CSS rule.