A schema determines which elements, attributes, and capitalization is valid in that schema. (It also determines the tags and attributes that will be available in IntelliSense.) Select this option if you want Visual Studio to use the content of the page's <!DOCTYPE> declaration and html element to determine the schema. For example, if you select this option and the page contains the declaration <!DOCTYPE html>, Visual Studio uses the HTML5 schema. However, if the html tag contains an xmlns attribute, (for example, <html xmlns="http://www.w3.org/1999/xhtml">), Visual Studio uses the XHTML5 schema.
Target when no doctype found
Choose the schema to validate against when there is no <!DOCTYPE> declaration in the page.
Select the check box in order to enable validation. If the check box is not selected, the editor will not mark validation errors.
The remaining check boxes allow you to fine-tune validation by specifying individual types of errors that you want the editor to mark.
Some schemas do not offer options to mark individual types of errors. For example, if you choose XHTML 1.1 as the target schema, all option check boxes are disabled, because in that case all types of errors are marked.