Have you run into a situation where the content editors of your Sitecore site are getting xHTML validation errors when doing simple things like adding links in the Rich Text Editor?

Rather than turning off the validation altogether, there is a way to reduce the strictness level:

1) Download this file xsd file for the xHTML transitional schema:

http://www.w3.org/2002/08/xhtml/xhtml1-transitional.xsd

2) Save the xsd file to this folder in your Sitecore website:

/sitecore/shell/schemas/

3) Add this Sitecore Patch to a config file:

4) Restart your website and you are good to go.