Whenever you're designing a website, one of your primary goals, besides communicating the mission of the organization for whom the website is being made, should be to make the website accessible to all visitors, no matter what kind of computer or browser they have, and no matter what kind of disabilities they have (whether it be blindness, deafness, or other problems).
Luckily for you, there's a free and easy-to-use tool on the web that lets you check how well your website conforms to coding standards:
The W3C generously provides this service to further their mission of having an open, accessible and free web. The tool is dead simple to use: just type in your website's URL, and click Validate. Errors will then show up, and you can go back to your source code and fix the little mistakes you've made. But there's a lot more about Validation that needs to be said!