It's all very well having a website that seems fine to you, on a wide-screen pc with high-speed connection and no special accessibility needs. But not everyone sees the web like you do, and search engines certainly don't.

Websites are interactive, with clickable links and submittable forms, and text can be in headings or paragraphs, with emphasis, etc.

All of these special characteristics are marked up in the code around each bit of content, behind the scenes, in a language called Hypertext Markup Language (HTML). Hypertext means "more than just text", because it has clickable words and all kinds of other stuff, like we've just discussed.

Matt Cutts explains Google's current priorities, including how W3C-compliance-based validitity is a good ballpark target but not identical to Google's internal standards for interoperability, and how non-critical validity is slightly less important than simplicity of code...

Validity

Make your code valid, by following the standards set by the World Wide Web Consortium, so that browsers don't get confused when deciding how to display your website. If browsers don't understand your code, they might give users something different to what you intended your site's users to see.

Critical elements

If your page title tag is not coded properly, it can have disasterous effects on your site's rankings in search engines. Some other elements in your HTML source code are also quite critical in this respect.

Noncritical elements

Try to code your whole site validly, for optimum SEO success. Every part of every page, in a validly standards-compliant way.

Efficiency

Simplicity

“Don't... make 1 word for every 200 blank lines, but otherwise... I wouldn't worry about it that much...” — Matt Cutts, Google

Try to keep your code short & sweet, using minimal code to achieve certain semantic results. This way you can optimise the speed & accessibility of your site, making big differences for people with very slow internet connections.

Avoid very heavy, long-winded client-side scripting & otherwise bloated markup code. This can slow down or even crash a low-performance PC or network. And the less simple your scripting, the more likely you're trying to hide something malcious.

Semanticity

Semanticity is about using the right code to outline which sentences are headings and which are normal paragraph text, and what words could replace your pictures for non-visual users (including search engines)...etc.

  • STRONGLY AVOID: Any JavaScript/DHTML/CSS that plays with critical visibility of properly marked-up elements. For example, avoid CSS like "visibility:hidden", "display:none", "position:absolute;left:-1000px;", "font-size:xx-small", "color:white;background:white", etc).
  • LIGHTLY AVOID: Unsemantic elements. Eg, use "<a>" for links rather than "<div onclick...>", and use "<h2>" for a second-level heading rather than "<b><font size...>" to distinguish the start of a new sub-topic, and use "<p>" rather than "<br>" to start a new paragraph; even though they may look the same on screen, because they mean different things to non-visual users (including search engine robots).

Avoid code that could be used for cloaking

See Google's policy on cloaking.

Avoid playing with visibility, especially like this example.

You might have a script that performs a cool function, but if it plays about with visual effects in a similar way to how spammers might do, then there's always going to be a trust issue.

Where as if you steer clear of all techniques that are strongly associated with fraud & deception, irrespective of how ethically you would be using them, and you provide all users (visual and non-visual, including search engine robots) with the same information via optimally marked-up semantic (meaningful) markup code, then search engines can immediately tick one box, without sandboxing (delaying), to say that there's absolutely no evidence whatsoever that you may be doing spammy tricks.

This is one more box that gets ticked, and one more step up the rankings. This is a major factor in futureproof SEO, and a standard concept in present-day, white-hat (ethical) search engine optimisation strategy.