Add
type="text/css"
to your link tag
While this may no longer be necessary in modern browsers the HTML4 specification declared this a required attribute.
type = content-type [CI]
This attribute specifies the style sheet language of the element’s
contents and overrides the default style sheet language. The style
sheet language is specified as a content type (e.g., “text/css”).
Authors must supply a value for this attribute; there is no default
value for this attribute.