[ibrahima]

I think it's more likely that they simply haven't felt the need to update their styling much in the past decade or so. And last I checked the home page uses tables for layout, so it would make sense that it works on IE5, since that's what everyone did back then.

[TazeTSchnitzel]

Actually, they did do a big styling change a year or two ago.

[futhey]

I feel like this is a more sufficient explanation than the one provided by the author. The front-end people at wikipedia probably remember the hoops they jumped through to work within IE5's quirks, and developed best practices that still happen to work out in that context.