This goes for outside of HTML too--most on-screen text can benefit from the "space, don't indent" ideology.
Anyone who's been on mailing lists or newsgroups awhile has probably run into messages like this:
Foo wrote:
> Lorem ipsum dolor sit amet, et adipiscing...
Amet!! Quiam dolor neniu ipsum, dolorem quia est.
Hamin qua other gibberish just to take up space! The
nerve of him! Lorem ipsum flame flame flame.
And more of the same, with other gibberish included
just to take up space.
This gets to be inherently unreadable, even more so because people who type like this tend to have irregular quoting methods as well--imagine that the > in the above message was left out--and it becomes, at worst, an interminable struggle to discover who wrote what, as well as being very uneasy on the eyes. Compare this:
Foo wrote:
> Lorem ipsum dolor sit amet, et adipiscing...
Amet!! Quiam dolor neniu ipsum, dolorem quia est.
Hamin qua other gibberish just to take up space!
The nerve of him! Lorem ipsum flame flame flame.
And more of the same, with other gibberish included
just to take up space.
Do everyone a favor: save your indents for print media. Space your paragraphs for the screen.