CSS - Naming Conventions

Hello,

In my HTML I name the Ids always with Capitalized first letter in each word:

Intro, LeftContent, ...

I do the same with the CSS:

Intro, LeftContent, BlueParagraph, ...

I see other variations like using _ between each word. Or capitalizing only the second word and forward.

What do you usually do? Is there a more common way than others?

Thanks,
Miguel