According to at least WebKit browsers (https://stackoverflow.com/a/8245664/122441): There are only 2 valid types for stylesheets: * text/css * text/xsl Currently mime.types maps .xsl to application/xml which in browsers gives a warning: "Resource interpreted as document but transferred with MIME type application/xml." (happens on Safari, Chrome; should happen with other browsers like Opera, and upcoming Chromium-based Edge) The proposal is to change .xsl to text/xsl. Original proposal: https://github.com/jshttp/mime-db/issues/150 Related to bug 55963