Depends on the features you're trying to implement - all browsers behave differently, particularly version differences between IE. For example, IE8 is over a decade old.
Maybe try starting at a site like this The HTML5 test - How well does your browser support HTML5? or W3Schools Online Web Tutorials to determine what features work on which browsers.
if the problem is with button colors etc you might have the same issue I recently had, check Theme 25 Color Modification Issue in IE
the clue is then in decoding all the base64 stuff in the css and also change those includes.