Inspired by this question, I was wondering whether this works:
Each creature that starts its turn in the webs or that enters them during its turn must make a Dexterity saving throw.
Whereas moonbeam states:
When a creature enters the spell's area for the first time on a turn or starts its turn there, it is engulfed in ghostly flames that cause searing pain, and it must make a Constitution saving throw.
And the description of the Ready action states the following (from PHB page 193; emphasis mine):
First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.
Assuming whatever trigger your readied action had does actually occur before the start of your next turn, can you escape from these spells?
It is possible for this to occur through chance. Say you had already readied movement, and then it happened that a creature cast moonbeam on you, and then your trigger (unrelated to moonbeam) occurred. Alternatively, if a creature readied (and thus cast) moonbeam and somebody identified the spell, you could ready movement as a cautionary measure to avoid taking the damage if the creature placed the moonbeam over you.
This is also similar to scenarios where an ally could allow you to move, such as the Battle Master Fighter's Maneuvering Attack Maneuver, or if an ally directly moved you, such as through grappling.