You can validate your website using this validation service. If you are using Opera, validating a website is just one click away : Right Click -> Validate.
Recently I have updated all my free wordpress themes to be 100% xhtml valid. After validating each of them, I found that there were some common errors, which can be solved by:
This is also known for improving your site’s SEO. Whenever you use <img> tags you have to add an alternate text. If you do not have alernate text to add, at least leave it empty like this alt=""
<img src="" />
<img src="" alt="" />
Tags like <img> , <br> , <meta> need to be self-closed.
Some other self-closing tags are:
One example, the meta verification tag given by Yahoo! Site Explorer (Yahoo webmaster tool) is always in uppercase and not self-closed, which make my page invalid.
You have to manually change the word “meta” to lowercase and self-close the meta tag.
Given by Yahoo! – invalid
<META name="y_key" content="xxxx" >
Edited to make it valid
<meta name="y_key" content="xxxx" />
By default, the embed codes given by youtube are not xhtml valid.
To make it valid, you can refer to the guide I wrote on last month.