I'm not quite sure where this goes since it involves the official client and also the game's actual engine. Anyway.
Right now, ST <mob> and tab-targeting in the HTML5 client interact in a counterintuitive way.
If you do ST CERBERUS and walk into a room with a cerberus and a cultist, you will be targeting the cerberus (and it lights up in the IH pane to make this clear). If you then start tab-targeting, the UI will indicate that you're changing your target to the cultist, but it doesn't actually change - when you attack, you'll hit the cerberus.
Solutions, from least desirable to most desirable.
3. At the very least, the UI shouldn't change.
2. Slightly better would be tab-targeting replacing an ST target (and vice versa). This would be a lot more intuitive than the current situation, though it isn't a very ideal solution since having to re-enter the ST target after the tab target is gone or dead is a pain.
1. The ideal solution would be to have the game maintain two target variables: an ST target and a tab target. If the tab-target is in the room, attacks should automatically target it. Otherwise, attacks should go to the ST target. This offers the benefits of both systems: you can ST CULTIST and not have to bother hitting tab for each cultist individually every time you want to kill one, but if you want to target a specific cultist or a cerberus you can tab-target to it, and once it's dead or you've moved, you're back to your "default" ST target without having to enter the command again. This would also be handy when fighting players - you can ST target the player, then tab-target to their ents and, once they're dead, you're back to targeting the player.