Whitespace
"Whitespace" is ignored by browsers. "Extra" spaces, tabs and newlines are all whitespace. Extra spaces are spaces, or more spaces than are required by HTML syntax or to separate words. In particular, only one space will be displayed between the end of one sentence and the beginning of another, no matter how many are in the HTML source.
Simple webpage with no extra spaces in the HTML source:
<html><head><!-- Metadata goes here in the "head" before the body element. --></head><body> The visible content of the webpage goes here in the body element. The body can also include images, and other elements.</body></html>
The spaces in the "Comment" (<!-- Invisible comment -->) are an essential part of the HTML syntax and are not "extra". Extra spaces there wouldn't do any harm though.
More examples of essential spaces:
<meta name="description" content="This is the description of the web page.">
<meta http-equiv="refresh" content="4 URL=~meta.html";>
<a class="top-menu" href="http://cybercoyote.org/classes/classes.shtml" title="Classes">classes</a>
