The most popular and trafficked websites are being redesigning using web standards. The three main reasons why it makes sense to use web standards are : 

Marketing Advantages

• Amazing search engine optimization: Natural search engine optimization (SEO) occurs as semantic markup eliminates frivolous table tags, font declarations and spacer images that are not part of the page's content. Search engines like Google, Yahoo, and MSN, are only interested in the content, and quickly index and favorably rank these pages.
• Fast to market: Beyond the cash savings described above, your business will benefit on several marketing levels. Using web standards enables rapid development. You can not only launch your client's site quicker, but also quickly update content.
• Rapid, site-wide changes for re-branding efforts

Cost savings

• Less development hours
• Fewer people are needed to redesign your website: Another long-term cost benefit is the lower maintenance expenses. Visual and structural changes can be made fast and efficiently with CSS. HTML and CSS are comparatively simple languages. An experienced in-house web developer can make required changes, wouldn’t incurring retainer fees with the original designer.
• Dramatically lower bandwidth costs: Using less code and images can easily cut bandwidth bills in half. 
• Simplified, fast maintenance: Any knowledgeable developer can understand and alter the code. 

Customer Advantages

• Pages load quicker and are more responsive: Standards-based markup streamlines the user experience. These are your customers, so you will receive extra benefits with no additional development costs. When pages load quicker and are more responsive, users are likely to spend more on the website. The second a layout breaks or takes too long to render, the visitor could click to another website. This is especially relevant to the crowd still on high-speed connections whose attention spans have a two-second threshold.
• Cross-browser and cross-platform compatibility: A standards-compliant website also ensures wide compatibility. It will render correctly in every modern browser on any platform, and degrade gracefully in older software. XHTML websites are not only future-proof against upcoming browser versions, but can accommodate other devices such as PDAs. Future website-wide redesigns will not be needed.
• Future-proof: Your website will be ready for the next gadgets or devices that will be invented for browsing the Internet. 
• Increased accessibility for disabled users: Semantic markup facilitates accessibility for disabled Internet users. While it is not a complete solution, it alleviates a large portion of accessibility issues that exist in non-standard websites, and is the first step towards meeting government guidelines for accessibility. The advantage is acquiring more loyal customers rather than incurring long and costly legal battles.