Making sense of CSS Lint warnings

I am writing the CSS for our website and I just ran it through CSS Lint. I'm struggling making sense of quite a few of the warnings so would greatly the community's assistance.

1. Don't use IDs in selectors.

   Isn't that the point of the IDs? To be used to address a particular element on the page?

2. 2 IDs in the selector, really?

   Is there a better way of selecting an element rather than using two selectors in the same line?

3. Broken box model: using height with border-top.

   I have no idea what this means. My understanding is that box height is separate to border height. I have defined a height for the element than then the border sides are being individually defined, where am I going wrong?

4. Heading (h1) should not be qualified.