What's the difference between an id and a class?

Answer 1

CSS selector space actually allows for conditional id style:

h1#my-id {color:red}
p#my-id {color:blue}

will render as expected. Why would you do this? Sometimes IDs are generated dynamically, etc. A further use has been to render titles differently based on a high-level ID assignment:

body#list-page #title {font-size:56px}
body#detail-page #title {font-size:24px}

Personally, I prefer longer classname selectors:

body#list-page .title-block > h1 {font-size:56px}

as I find using nested IDs to differentiate treatment to be a bit perverse. Just know that as developers in the Sass/SCSS world get their hands on this stuff, nested IDs are becoming the norm.

Finally, when it comes to selector performance and precedence, ID tends to win out. This is a whole other subject.