Know where to find documentation!

A few sites you should know:

https://developer.mozilla.org/en-US/
Covers pretty much everything there is to know about a browser.
For quick lookup type "mdn <any javascript method>" in google

http://caniuse.com/
Compatibility tables for support of HTML5, CSS3, SVG and more in desktop and mobile browsers.

http://www.quirksmode.org/compatibility.html
Dealing with old browsers, css and javascript