2.2.18 [HTML] Section 12.3.2, Links and external style sheets
The specification states:
When the LINK element links an external style sheet to a document, the type attribute specifies the style sheet language and the media attribute specifies the intended rendering medium or media. User agents may save time by retrieving from the network only those style sheets that apply to the current device.
All Document Modes (All Versions)
All style sheets are retrieved whether or not they apply to the current device.