Both browsers obey this tag but Explorer users have the ability to change their font size settings from within the program. There are two ways to approach this problem. First, allow users to set their font to whatever size they want and just make sure your design is not dependent on specific placement of page elements like photos and text areas. This solution limits you to horizontal design. It's not so bad on a page like the one you're reading now, the text just gets larger or smaller and the page grows or shrinks in length.
Most newer browsers do.
Use these commands with care because different computer systems render fonts differently. My Macintosh 12 point text is equivalent to a Windows machine's 10 point text. Not all browsers support Javascript, but most do. That's the lesson. It is wise to view your design on different computers and with different browsers.