In game contexts, you can only do things specifically allowed by the rules.
Thanks Donald.
Does this also hold for denominations? For example, I play a Pawn and say I use it to get "+1 action, +1 purchase" instead of "+1 action, +1 buy". Humans will know what I mean, but is it technically correct? Similarly, the rules only say that the player with the most VP wins (or tied with fewer turns, or tied). Does that mean there is no 'official' second place?
Exactly. It's a rule that's implicitly written in every rulebook for every game, but MTG felt the need to spell it out. I suppose you could say it changes the reasonable expectations of a game... in most games, if a player who was losing simply gave up and left, it may be considered rude. People do it in Online Dominion all the time, but it doesn't generally happen in IRL Dominion.
Yes, resigning would be a good example of what I'm talking about.
There is no rule on resigning in Dominion (IRL). So technically the game would stall once the player who 'left' has a turn.
But perhaps, within the rules, you could 'resign' by saying that on every turn, you do absolutely nothing. Then tell the other players what they should do with your cards when an interaction/attack is played (e.g. 'If someone plays a Torturer, always discard the leftmost two cards'). By doing this, the other players can keep playing, and simply perform your turns by only doing your clean-ups (I assume that players are allowed to do stuff with your cards if formal consent is given, such as shuffling your discard while you visit the bathroom).
Or you can use house rules for resigning. (of course you could quit mid-game because humans are playing it and not robots - I'm taking a formal approach here)