Multi-line comments in CSS follow the C style of commenting. The beginning of the block is indicated by a /* and the end of the block is indicated by */. For example, in the following block:

#header {
  /* margin-top: 10px;
  margin-bottom: 10px;*/
  float: left;
  clear: both;
}

the margin-top and margin-bottom lines will be ignored. Easy enough. In practice, it's quite easy to make the unintentional mistake of nesting comments inside a block of already commented code. Taking the above example, we may want to disable the entire #header block of code, say for debugging purposes. It's tempting to do the following:

/*#header {
  /* margin-top: 10px;
  margin-bottom: 10px;*/
  float: left;
  clear: both;
}*/

Unfortunately, the resulting code will be anything but predictable. The CSS specification is very explicit on nested multi-line comments and how they are NOT supported, just like with C comments.

Comments begin with the characters "/*" and end with the characters "*/". They may occur anywhere between tokens, and their contents have no influence on the rendering. Comments may not be nested.

When comments are nested, the nested comment's beginning delimiter /* is ignored yet the closing */ is not. Basically the larger comment ends with the nested comment's closing delimiter.

The problem is that when you're debugging CSS code, it's very easy to believe that there's a problem with the rendering engine rather than a problem with your CSS code. That's because IE and Firefox, the two most common browsers, have numerous bugs or unexpected behavior with respect to CSS rendering.

When you've spent hours trying to figure out a workaround for why that block of code is just not floating the way it should, check to make sure your code does not have these unintentional nested comments. Yes, you will be kicking yourself (as I've done on at least a couple of occasions) but at least you found out what went wrong.