Strictly speaking, it should be 'a mtb website' because you should write sentences that include abbreviations as though the term was not abbreviated. You would not write 'an mountain bike website' so you shouldn't write 'an mtb website'.
I'm with NZCol on this one. The standard of writing is getting worse and worse. Exam results might be getting better, but what about the level of knowledge and skill that kids are actually gaining?
One of the reasons that I've done as well as I have in my career is because I can write correctly, producing well thought out and articulated documents. In every company I've worked in, this has been remarked on as a rare skill which, while flattering, I do find to be rather depressing.
Do not get me started on something as basic as the correct use of the words 'there', 'they're' and 'their'.