How good will HTML5 games run?

Rugus asked me to cover the topic of HTML5 games, as he was very impressed with Runescape 3. As I previously wrote, I do believe that making Flash games is a mistake. Companies that exclusively rely on Flash games like Zynga, and Facebook which offers a platform for those Flash games, are both being financially hurt by the trend of people moving away from PCs and onto iOS or Android tablets. So you end up with a game which from its content is close to optimal for playing on a tablet, but which

World of Warkraft...