[youtube https://www.youtube.com/watch?v=WD93tBGimpo]A few years ago, checking that the site worked in the two main browsers (Firefox and Internet Explorer) was all that mattered; the validity of the markup was not deemed important. You will still hear people talking about “both browsers” as if nobody ever used anything other than Internet Explorer or Firefox, but this is a very outdated attitude.There are so many different devices now (such as PDAs, tablets, smartphones, screen and Braille readers for the visually impaired, and a variety of alternative web browsers) that building a site for each of them would be impossible. Even if you managed to cover all devices currently in use, something else will be released next week. Your best hope is to comply with the standards.
Future-Proofing
The latest generation of web browsers (Internet Explorer, Chrome, Firefox or Safari) are the most standards-compliant ever. Browser manufacturers have realized that standards are a good thing.
Back in 2000 when Netscape released Netscape 6, they dropped backward-compatibility for many of the hacks and proprietary code that had been developed for their version 4 browsers, in favor of developing a browser that displayed code written to comply correctly with the W3C recommendations.
This phasing out of deprecated markup could easily happen again with mainstream browsers, as they continue down the path of compliance with the W3C recommendations. So if you write non-standards-compliant markup, it may not be readable in future browsers.
Additionally, many of the presentational tricks in common use today are exploitations of browser behavior. There is nothing to say that a newer version of the browser will behave in the same way. Validating your document will give you peace of mind knowing that the latest and greatest version of Internet Explorer, Chrome, Firefox or another browser won’t leave you with a broken web site.
Accessibility
Accessibility is becoming a legal requirement in many countries. Even in those where it is not, ensuring that everyone can use your site is an excellent aim. Should laws be applied to the organizations you are creating your sites for in the future, you will not need to begin rebuilding in order to comply with them.
Complying with web standards will assist you in ensuring that your web site is accessible to those with disabilities. Though you can still create an inaccessible site when adhering to the standards, compliance with the standards does, at the very least, offer a greater chance that devices such as screen readers and Braille readers can interpret the markup correctly and make your content understandable. With a solid base of valid markup, you will be on a good footing to ensure that your site is fully accessible to all web users.
Accessibility need not just apply to users with disabilities—those using limited devices such as PDAs and web phones will also benefit from clean, compliant code, as will search engine robots (which understand the standards and use them to help them spider and index the content of sites and decide where in the ranking a site should be placed).
Ease of Debugging
As we have already mentioned, standards-compliant code can be far quicker to debug. Often, by simply validating and fixing errors that are flagged up, you can isolate and correct a display issue. Even if the process does not correct the problem, you will be well placed to work out where the problem lies.
If you post to design- and development-related mailing lists with a problem, you are likely to get a better response if you can demonstrate that you have validated your markup and that this has not corrected the problem. It also makes it easier for someone to look over your code in order to give you some help and advice on the issue, so you are more likely to receive helpful replies to your queries.
Working As a Team
Working to the standards also makes it easier for other people working on your project, or taking on the project after you, to pick up where you left off. Many designers create HTML pages that will be used as templates for a content management system or other dynamic site. By creating templates that conform to standards, you will make it far easier for those who then need to incorporate that design into server-side code. You’ll also enhance your appearance as a professional who knows what he is doing—neat, valid markup looks far more impressive than a tangled mess of incorrectly nested tags and proprietary code.
