As mentioned, for efficiency reasons sequential spaces are consolidated into one space the browser actually displays. Remember what the ML in HTML stand for. It's a Mark-up Language, designed to control how text is displayed.. not whitespace :p
Still, you can pretend the browser respects tabs since all the TAB does is prepend 4 spaces, and that's easy with CSS. either in line like ...
<div style="padding-left:4.00em;">Indenented text </div>
Or as a regular class in a style sheet
.tabbed {padding-left:4.00em;}
Then the HTML might look like
<p>regular paragraph regular paragraph regular paragraph</p>
<p class="tabbed">Indented text Indented text Indented text</p>
<p>regular paragraph regular paragraph regular paragraph</p>