From Hacker Writing Style
of The Jargon File
The underscore is also common, suggesting underlining (this is particularly common with book titles; for example, "It is often alleged that Joe Haldeman wrote _The_Forever_War_ as a rebuttal to Robert Heinlein's earlier novel of the future military, _Starship_Troopers_.")
Over the years we have gotten used to the use of '_' around words to
show underlining. Something was needed for it on pure text messages
that otherwise have no way of displaying underlining (and is still
used for such things as SMS).
However, this is not the case on the web - there is an HTML tag for
that exact purpose. This tag, the humble <u> will make a
word underlined - just like that.
When writing for a medium, it is important to use and understand what
is available in that medium. While old habits die hard, it
/is/ is important to let go of them. Please
do your part in keeping web authoring separate from email writing - they
are two different media.