Over at Pharyngula, people regularly ask how to format posts in particular ways. Here's a rough guide to the half-dozen most common things people want to do.
In all cases, you use HTML (or HTML-like) tags, which (with a few exceptions) occur in opening/closing pairs with your text in between, like this (for an imaginary tag “foo”): <foo>some text</foo>. You have to be very careful to get the syntax right or you will get unexpected results. Preview is your friend.
Quoting
In order to quote a previous text, use <blockquote>. If you write
“<blockquote>John McCain is a tool.</blockquote>”, it comes out looking like this (indented and possibly with other decoration, such as a bar down the left):
John McCain is a tool.
Italics
You can get italics with <em> or <i>, like this: “John McCain is <em>definitely</em> a tool” produces “John McCain definitely a tool”.
Bold
You can get bold with <strong> or <b>, like this: “John McCain is <strong>definitely</strong> a tool” produces “John McCain definitely a tool”.
Strikethrough
You can strike text with <s>, like this: “John McCain is <s>not</s> a tool” produces “John McCain is not a tool”.
Underline
You can underline text with <u>, like this: “John McCain is <u>such</u> a tool” produces “John McCain is such a tool”.
Links
You can produce links with <a>. The text to be linked appears between the elements, and the URL appears in the href attribute. “<a href="http://www.google.com/">Google</a>” comes out as “Google”. Note that the quotes around the URL and the http:// are not optional. Also, including more than a certain number (2 or 3) links may result in your comment being held for moderation as an anti-spam measure. Preview is your friend.
Physical vs. Logical Markup
Not that it makes much difference in the tag soup that is the blogosphere, but to satisfy the pedants, I should note that the <em> element is preferred for emphasis and <i> for non-emphatic italics. Similarly, <strong> is preferred for strong emphasis and <b> for non-emphatic emboldening.
Who is that who prefers <em> and <strong>?
Pharyngula is very unusual in requiring <s> for striking stuff through. The normal HTML tag -- which does not work in Pharyngula! -- is <strike>. Also, neither of these tags works in any other ScienceBlog.
Also, it should be mentioned that if you want < and > to appear, you have to spell out their HTML codes: < and >.
Hi David,
Nice of you to drop by...
Way back when, <em> and <strong> were recommended because they were semantic markup rather than physical markup and, in principle, allowed screen-readers and other adaptive technology to render the emphasised text appropriately to the output medium. “Italic” and “bold” have no meaning in speech, whereas “emphasised” does: a screen-reader might read emphasised text in, say, a slightly higher pitch, whereas text italicised for other reasons, as we do with phrases like sine qua non, shouldn't be regarded as emphasised for the purposes of rendering to speech.
Nowadays, it makes very little difference because stylesheets allow fine control over all output media, but some people still maintain the distinction in HTML. I'm not anal about it, since the 'net is an ocean of tag soup in which my markup is but a dribble, but I do tend to stick to this old rule-of-thumb myself.