What are the advantages of using web standards?

Accessibility

To software/machines
Complying with web standards can give your web pages greater visibility in web searches. The structural information present in compliant documents makes it easy for search engines to access and evaluate the information in those documents, and they get indexed more accurately.Because use of web standards makes it easier for server-side as well as client-side software to understand the structure of your document, adding a search engine to your own site becomes easier and gives better results.Standards are written so that old browsers will still understand the basic structure of your documents. Even if they can’t understand the newest and coolest additions to the standards, they’ll be able to display the content of your site. The same, of course, applies to robots - systems that collect information from your site on behalf of search engines and other indexers.Compliant code gives you the opportunity of validating your page with a validation service. Validators process your documents and present you with a list of errors. This makes finding and correcting errors a lot easier, and can save you a lot of time.Compliant documents can easily be converted to other formats, such as databases or Word documents. This allows for more versatile use of the information within documents on the World Wide Web, and simplified migration to new systems - hardware as well as software - including devices such as TVs and PDAs."

To people

Accessibility is an important idea behind many web standards, especially HTML.
Not only does this mean allowing the web to be used by people with disabilities, but also allowing web pages to be understood by people using browsers other than the usual ones - including voice browsers that read web pages aloud to people with sight impairments, Braille browsers that translate text into Braille, hand-held browsers with very little monitor space, teletext displays, and other unusual output devices.
As the variety of web access methods increases, adjusting or duplicating websites to satisfy all needs will become increasingly difficult (indeed, some say it’s impossible even today). Following standards is a major step towards solving this problem. Making your sites standards-compliant will help ensure not only that traditional browsers, old and new, will all be able to present sites properly, but also that they will work with unusual browsers and media.
Some consequences of ignoring standards are obvious: the most basic consequence is that you will restrict access to your site. How much business sense does it make to limit your audience to only a fraction of those who wish be a part of it? For a business site, denying access to even small portions of a target audience can make a big difference to your profit margin. For an educational site, it makes sense to allow access not only to affluent, able-bodied school-children with graphical browsers, but also to children in regions with poorly-developed infrastructure who are best served by text-only browsing, or disabled students using specialized browsers.
The same principle applies to all types of websites — while straying from the standards and taking advantage of browser-specific features may be tempting, the increased accessibility which comes from standards-compliance will lead to far greater rewards in the long run.

Stability

Most web standards are generally designed with forward- and backward-compatibility in mind — so that data using old versions of the standards will continue to work in new browsers, and data using new versions of the standards will “gracefully degrade” to produce an acceptable result in older browsers.
Because a website may go through several teams of designers during its lifetime, it is important that those people are able to comprehend the code and to edit it easily. Web standards offer a set of rules that every Web developer can follow, understand, and become familiar with: When one developer designs a site to the standards, another will be able to pick up where the former left off.

Conclusion

As web developers, we are constantly trying to address the problem of inconsistencies between the renderings of web pages by different browsers and browser versions. This necessitates either time-consuming double/multiple coding, or coding for a single browser which makes it harder, if not impossible, for some of the public to use the site. This situation will be made even worse with the advent of additional hardware and software which will be able to browse the Web, such as telephones, pagers, and PDAs.
Web standards are not arcane laws decreed by ivory-tower organizations. As we have described, the standards are for the most part decided by representatives of the same people who use them — browser makers, web developers, content providers, and other organizations.
Writing web pages in accordance with the standards shortens site development time and makes pages easier to maintain. Debugging and troubleshooting become easier, because the code follows a standard. No longer do you have to worry about the coding and maintenance for several versions of code that are supposed to accomplish the same presentation. One version of your site, and that is it.
The universal adoption of web standards is becoming of paramount importance. The mission of The Web Standards Project is to make the Web a better place, for developers and for end-users, by encouraging browser and web page editor makers to follow the standards in their applications. This effort will be greatly helped when web developers use the standards as a matter of course, and insist that generators and renderers of their code comply with the standards.
The reasons we have given should give you, the web developer, plenty of incentive to begin using standards, and also plenty of ammunition with which you can encourage your place of business and fellow developers to use those standards.