This node isn't for
HTML,
XHTML,
XML,
CSS, or any other markup language: it's for advice on what looks pleasant. For example, the fact that bright pink and luminous green aren't a likely combination to be seen worn on the street, due to the fact that most people dislike them when in combination with eachother (or so I've been told.). The same principle applies with web pages: if you happen to use foul colours, fonts, and bad layouts, people don't like it, and that decreases your chances of people visiting your page.
Making pages that're downright hypocritical is bad too. For example, if you were to buy a shirt which has the words “
Capitalism sucks” on the front, and yet you shelled out $30 on it, that would defeat the object of the shirt. A similar concept would be making a lovely graphical page to advertise a textual game, such as a Multiple User Dungeon (
MUD for short), or a normal Text Adventure (Also known as an “
Interactive fictional program”). This would be hypocritical due to the textual nature of the games being advertised by fancy graphics, which have absolutely nothing to do with the content whatsoever. Trying to lock the design in with the content is essential, as this either emphasises the content's message, and is often seen as more friendly than badly formatted tables or slightly off subject graphics – or indeed, badly made graphics.
Colours that're similar are generally good: various shades of blue which go together (usually darker for the background, lighter for the foreground/tables/mouseovers and so on). If you as the designer enjoy just looking at your page, you may well be onto a winner.
Last but not least, however, has to be the markup you use. Invalid markup is awkward for some: sure, maybe the majority of people who will/have viewed your page will use a browser that can handle the occasional tag not ending and so on, but there are always browsers that won't enjoy (I.E., will crash) rendering bad tags. Admittedly this is rarely HTML, but usually Javascript, CSS, or something along those lines, but it does happen. One example I myself have encountered is the problem with opacity on various browsers: Internet Explorer will render opacity and not crash – though it will run badly, it won't crash. On the other hand, Mozilla for example, should it be the “wrong kind” of opacity, will crash upon navigating around a site – as I say, I myself have encountered this problem, so I would know.
Note: to any americans reading this, excuse my British spellings such as “colour” (as opposed to “color”) and so on.