So if I don't want any space between the paragraphs, I can delete whole height 1em part? And then search and replace bigger ems with br/?
I believe the <p height="0"> is
how you achieve no space between paragraphs. If you just used <p>blah</p><p>blah</p>, I think that will result in block (no indent) paragraphs with a space between the two paragraphs. I don't think there's any difference between <p> and <p height="1em" width="0">.
And <br /> will only ever get you to the next line. You can't "stack" them to make a larger space.