Why

There are several good reasons for Web publishers to ensure that their sites are standards compliant, do not assume some specific browsing environment, provide meaningful semantic markup where possible, and are not dependant on javascript, CSS, Flash, etc. They are:

1. accessibility
2. cost
3. indexing
4. development of the Web as a whole
5. legal requirements

Accessibility

Contrary to popular belief, accessibility applies to any type of access, not just to people with disabilities. When access to some content is made dependant upon some technique that cannot be relied upon to be available (javascript, CSS, Flash, PDF, any specific Web browser, etc.), accessibility is degraded. When DIV soup replaces semantic mark-up, accessibility is degraded. When things like font-size are not left up to the user, accessibility is degraded. When only access through large screen/high bandwidth desktop computers is anticipated, accessibility is degraded.

Cost

A site depending on user-agent quirks is more costly to build. It requires continuous research of those quirks; identifying them and finding workarounds that don’t introduce new problems. It requires even more maintenance to ensure that those workarounds don’t cause problems in new browsers, which may not have the same quirks.

A site that uses invalid code, relying on browsers to guess what was actually meant, is likely to ‘work’ differently (not work) in another browser and will thus require more resources to build and to maintain (keep it ‘working’) in other, both current and future, browsers.

A standards-compliant site on the other hand is cheaper to build and is cheaper to maintain, as it will simply keep on working in newer browsers. It will therefore also create less problems for users, and thus lower helpdesk costs.

Indexing

Finding resources requires those resources to be indexed. This is what search engines do. But in order for a search engine to index a site well requires that website to be clear about what it contains. This is where proper markup (HTML) comes in. HTML is not just about structure, but also very much about semantics. The better a Web Publishing System is at outputting semantic markup, the better the resulting website can be indexed, and thus the easier it can be found by the audience it aims to reach.

Development of the Web

Unfortunately the current reality of the Web is that even when a site is broken (not standards-compliant) users still expect their Web browser to make sense out of it. The result is that browser developers need to spend a lot of resources on development of code to make their browser guess what was meant; resources they could invest in furthering the evolution of their product, and thus the Web as a whole, if the majority of Web sites were not broken. Thus, the more broken Web sites out there, the less the Web will evolve. That’s just plain bad economics.

Legal requirements

A growing number of countries has legislation that requires Web sites to be accessible. Whether such legislation is aimed only at people with disabilities or more general, it always means that a site can rely only on valid, well-formed, semantic HTML.