If I create an interactive maze game with html5, would people be able to play the game on all platforms? Also, is it true that you can't play a game created by html5 on internet explorer?
Look at a programming language called haXe and a library for it called NME.
Have a look at RiaStats for a general idea of HTML 5's market penetration. The canvas element, which can be used for 2D games, is supported on all the latest browsers and most mobile platforms (most being any new devices or devices running the latest OS). You can run 2D HTML 5 games in IE 9, but not any of the previous versions. People still on IE 8 (released with Windows 7) or IE 7 (WinXP) without upgrading will not be able to run HTML 5 content. You can display a friendly error to them however, letting them know technology is evolving and they should tag along
On top of what TheNut just posted, be aware that there is no "standard" for 3D in html 5
You have the canvas for 2d, which is supposed to be standard (haven't done any exhaustive testing), but the only cross platform solution at the moment is webgl.
However webgl is hardly supported by anything at all
"HTML 5" has become a very loosely defined set of techniques. It generally means stuff that recently became possible with modern browsers. Thus, you can't expect any of it to work on older browsers, like IE 6 or even FF 3.0.
Also be aware that "all platforms" really is a lot of platforms these days, even if you just count the modern ones. You might want to make sure it works on touch-devices, which means hovering can't be used for input.
There is also a wide range of performance on the web. Anything from cheap Android phones to high-end desktop PC:s.