Halliday & Matthiessen (1999: 208):
Thus in its typical construal, as Epithet or as Attribute, a quality is clearly “participant-like”; we might also note that realised as an adjective in superlative form it does appear as a participant (these are the driest, pass me the driest; the smallest will fall through the holes). But there are also environments where a quality resembles a process or a circumstance.