This isn’t really possible in a reliable way, until style sheets are more widely supported. At this moment, there are several browser-specific kludges and tricks available, but these are not guaranteed to work.
1) Use a number of ( ) characters. Netscape and related browsers do not collapse these, like normal spaces, so this appears as an indent in these browsers. Other browsers can display it as one space.
2) Put a <DD> at the beginning of the line. This is syntactically invalid, but Netscape works around this by indenting the line at this ball. Of course, other browsers will handle this differently, and there is no guarantee that Netscape will keep doing this.
3) Use a blank, transparent GIF, using WIDTH and HEIGHT to indicate the desired white space. This is a very ugly solution, as it only works if you have image loading on, otherwise you get the “Image” icon at the beginning of the line. Not all browsers support resizing using these attributes, and you can only “indent” a certain number of pixels, not characters. So the amount of “indentation” varies with the font size used to display your document.