> Is this style common in Haskell Web development?

Not at all; the author is quite obviously inexperienced in using Haskell, and is using some weird old crufty library to create HTML.

The two most common template libraries are Heist[1] and Hamlet[2]. Heist uses Rails/Django-style external template files, while Hamlet embeds markup directly into the Haskell source code using quasiquotation.

[1] http://snapframework.com/docs/tutorials/heist

[2] http://www.yesodweb.com/book/templates

Even if one wanted to represent markup as Haskell types, there's no reason it needs to be as ugly as in that example. A really basic version might be:

  div [("id", "gallery")] $ do
    div [("class", "screenshots")] mzero
    ul [] $ do
      li [] $ do
        a [("href", "view.fcgi?file=" ++ first)] mzero