Naming "class" and "id" HTML attributes - dashes vs. underlines

It really comes down to preference, but what will sway you in a particular direction might be the editor you code with. For instance, the auto-complete feature of TextMate stops at a hyphen, but sees words separated by an underscore as a single word. So class names and ids with the_post work better than the-post when using its auto-complete feature (Esc).

I use the first one (one-two) because its more readable. For images though I prefer the underscore (btn_more.png). Camel Case (oneTwo) is another option.

Actually some external frameworks (javascript, php) have difficulties (bugs?) with using the hypen in id names. I use underscore (so does 960grid) and all works great.

I would suggest underscore mainly for the reason of a javascript side-effect I'm encountering.

If you were to type the code below into your location bar, you would get an error: 'example-value' is undefined. If the div were named with underscores, it would work.

javascript:alert(example-value.currentStyle.hasLayout);