While technical writers and devotees of the Chicago Manual of Style typically use serial commas in their writing, there are good reasons for Web writers to follow the Associated Press Stylebook, which advises against them.
First, to better engage readers, Web writing is usually more colloquial and journalistic in style, which is not in line with the more formal serial comma. But there’s a more important reason for not using them. In many languages (e.g., French, German, Italian, Spanish), the serial comma is not the norm and may even go against punctuation rules. So if your site will be translated into another language, your serial comma may be lost in translation and come across as unpolished. That being said, a serial comma is always welcome if it’s purpose for joining the party is to avoid ambiguity or confusion, or if an integral element of the series requires a conjunction.