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Link to original content: https://www.w3.org/2007/10/test-accept-header
If the browser sends only text/css for style sheets, this sentence should have a green background, otherwise it will be red.
If the browser doesn't send text/html nor application/xhtml+xml when requesting an image, the following image should be a green square, otherwise, it would be a red square.
(this test was developed in the context of a comment on mobileOK basic.