I can't change size of h1 with font-size property in c9.io editor

Question

As I mentioned in title, I'm using c9.io for hosting one of my projects. For some reason, I can't change h1 size with font-size property in CSS. I tried pixels, percents, em etc. but none of them worked. Surprisingly, if I tried using font-szie in code editors like Notepad++ or VSC on my PC, this property would be applied to h1, but c9.io's editor is different. Why is that? Thanks for answers in advance!

PS: I've been looking for simillar threads here on stackoverflow, but as I said, their solutions worked on my local editors only, so please, don't mark my question as duplicate of other one. Oh, and here's my CSS:

h1{ 
 font-size: 3em;
}

Answer 1

Well, I asked my colleagues from Udmey course chat, and one of them told me to use !important after the value. And it works, here's how the CSS looks like now:

h1{ font-size: 3em !important }