I’ve just created a CSS Menu on my site www.la-matha.com . It works fine in Chrome and Firefox, but the cascading items do not appear in IE10. I’ve seen very old items on forums as this was an issue with hover in earlier versions of IE, but I got the impression they were resolved.
I am a novice, with no programming background, so forgive the crudeness of my coding!
Hi Welcome to Sitepoint 
You are using an old partial doctype (without uri) which triggers quirks mode in all versions of IE and will make it behave much like IE5 and forget anything its learned in this century. It’s a trap especially put there for newbies to fall into.
The correct transitional doctype is:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
But these days you should really use strict but that means you can’t use old deprecated elements.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
You can easily check what mode IE is running by pressing f12 and looking at the developer tools which will tell you if the page is running in quirks mode or standards mode. You almost never want to run IE in quirks mode (despite what anyone else may say).
That was quick, simple and efficient. Thank you!