I enabled the X-XSS-Protection header in the web.config file:
<httpprotocol>
<customheaders>
<remove name="X-Powered-By" />
<add name="X-XSS-Protection" value="1; mode=block" />
</customheaders>
</httpprotocol>
Now the X-XSS-Protection header appears in responses for text/plain, application/json, text/xml or in requests for images, for example, but it does not appear in the response of the main text/HTML document. However, it seems that this X-XSS-Protection header is needed precisely on the text/HTML document. Do you have an idea why this might be happening?