CTRL + "/" toggle statements into comments and vice versa.
Brackets is written in JavaScript, HTML and CSS.
When editing HTML, use the Cmd/Ctrl + E shortcut to open a quick inline editor that displays all the related CSS. Make a tweak to your CSS, hit ESC and you're back to editing HTML, or just leave the CSS rules open and they'll become part of your HTML editor.
When your cursor is on a CSS rule, Brackets will highlight all affected elements in the browser. Similarly, when editing an HTML file, Brackets will highlight the corresponding HTML elements in the browser.