A while ago I implemented an algorithm from Wikipedia. It turned out it had a subtle bug. A single less-than instead of a greater-than. Unfortunately I spent a long time finding the problem, but it did start me thinking about how to improve pseudo-code on the web.
In an interactive medium there is little reason why algorithms should still be presented as static text. Text which can not be visualised, verified or checked in any way. Algorithms are complex beasts and ensuring they are correct is tricky and understanding them takes time. Running an algorithm is essential for correctness and stepping through an algorithm is extremely helpful for comprehension.