So a quick question about uploading files to the webhost?

I had a recent dilemma uploading some code to my webhost. I worked out the issues, but I am a little confused. It may be my fault seeing as I'm a noob. But here's something weird that happened. If I'm mistaken please tell me.

So I uploaded some code. Instead of making a div for the header I just used header on my local. Everything worked fine, but when I uploaded the code it would not accept header and basically deleted the code. The fix was creating<div class="header"> and that solved the problem.

But I was under the impression that header would work especially if I have at least one element of h1 -h6. I was using H1 for a logo and the UL for some social icons to float on the right side of the page. That was all I was trying to accomplish. The nav bar was going underneath.

Did I use the wrong element? Little confused on this one. Should I have just put header under the div class header selector. Or should I have just made div class=header?

Anyone got some insight. Did I screw that one up?