Okay, if the eyes didn't fall out, it means I need to add more.
If you have a choice, use HTML "5" (as a "living standard", it no longer has a version number officially). I don't mean the gee-whiz stuff like the <canvas> element, or the <audio> and <video> elements. Use 'em if you need 'em, but they're not relevant to SEO. I'm not talking about JavaScript wizardry or CSS 3 effects either. I mean use the new doctype...
... and the new content tags, particularly <hgroup>, <header>, <footer>, <section>, <article>, <aside> and <nav>.
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(This section deals primarily with long-form content. That would include essays, news articles, in-depth reviews, tutorials, etc. — things that generally involve scrolling more than one screen.)
There is a specification in HTML for document outlines (similar to the Outline view in a word processor like Microsoft Word, or to a table of contents). You might want to take a look at the relevant article over at
HTML5 Doctor, and make use of one of the outlining tools available (like the
Chrome extension).
It's better, when creating content, to work by starting with an outline and working towards content, but sometimes you'll already have long-form content to begin with. It may be something that someone else has submitted, or it may have been a bit of a "word doodle" that accidentally turned into an article/essay while you were writing. Search engines attach a much higher "relevance" score to things that are important enough to be headings, and the easiest way to see how a search engine index "sees" your page is to take a look at the page outline. If your outline looks like a decent summary of your article, the page will score a lot higher (all else being equal) than a page having the same content hidden in running text. (At the same time, it make the content a lot easier for a human reader to deal with. As I said previously, good modern page-level SEO and accessibility are nearly the same thing.)
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So, content is King. It always has been, really, but there used to be effective ways to lie about content (the keywords meta, JS content replacement, invisible text, and so on) that don't work anymore.
But there's one other really big thing left to consider: users will only take as much frustration as they think your site is worth. Click-throughs matter if you want to retain or improve your position, and blocks (which come mostly from the Chrome Personal Blocklist extension) can take you down quickly*.
If all else is equal (the same content marked up in the same way), a better-designed site will score higher over time than a poorly-designed site. It's not just that organic links will build eventually, but that people won't dread clicking on your link in the search results, and they won't tell their friends to avoid you. Positive word-of-mouth is great when you can get it, but it's just as important to avoid negative word-of-mouth. If your site is pleasant to use
and has reliable/valuable content, you'll get more click-throughs. And that will take you higher in the search rankings.
Typography matters. Graphic design matters. Layout matters. Navigation matters. Usability matters. And they all matter because user optimisation
is search engine optimisation over the long run.
I know: everyone wants a short-cut to the front page. But there are only ten unsponsored results on the front page, and eleventy-zillion sites all hoping to be there (some actually hoping to be there no matter what you're searching for). Start by being better than 99.9% of the crowd. That will put you just behind the established names. If you're good enough, you will become an "established name". And if that's just in a narrow sphere, that's okay—you won't be wasting anyone's time or fanning the flames of anyone's ire by popping up when you have nothing relevant to add.
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* There is a threshold for blocks. There are always oddballs in the crowd, so a few outliers won't hurt. And Google is pretty good at spotting organised "astroturfed" blocking, so it's hard for a competitor to take you down that way. ("Astroturfing" is a fake "grass roots movement", usually commercially-motivated, but sometimes politically-motivated as well. But then, who can tell the difference between commercial and political anymore?)