I've noticed that if I use
<a href=...></a> with no text or images in between, wordpress counts it as non-empty space, compared to if the same code typed in a regular html page with no wp involved, and wp pretty much considers it as a break line or something like that, which is nasty.
For example, I'm using the lightbox plugin and rather having multiple images displayed in my portfolio, I want to have only one followed by empty
<a> links, and giving each of those links the same
rel="lightbox[flowers]". and the extra
<a>'s are invisible but unfortunately made "visible" by wordpress.
Here's how it would look like: