You can remove the rule validation on the Global Rules item `/sitecore/system/Settings/Validation Rules/Global Rules`

You'll want to remove the rule from the `Validate Button`, and `Validate Button` fields of the item.

[![enter image description here][1]][1]

Update
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There are field specific rule items that you can update to remove the  `Is XHtml` if needed. The Rich Text field is located at `/sitecore/system/Settings/Validation Rules/Field Types/Rich Text`


  [1]: https://i.sstatic.net/oq4x2.png