As mundane as a document may be, though, the question seems to be whether the average browser is really going to care enough about how any given document looks to put in the effort to alter its presentation. If one already knows how to do so, then it's not so much an issue. Jeff I found two parts of the text very relevant to the question. The UA is free to choose the mechanism for referencing personal style sheets. Sometimes conflicts will arise between the style sheets that influence the presentation.
Conflict resolution is based on each style rule having a weight.
By default, the weights of the reader's rules are less than the weights of rules in the author's documents. I.e., if there are conflicts between the style sheets of an incoming document and the reader's personal sheets, the author's rules will be used. Both reader and author rules override the UA's default values. If each person is able to be a publisher then the reader is supported. The authors have given up the exclusive power of presentation.