2016-06-08 : Activated abilities contain a colon. They’re generally written “[Cost]: [Effect].” Some keywords are activated abilities and will have colons in their reminder text.

2017-03-14 : Arachnus Web’s triggered ability checks at the beginning of each end step, not just yours.

2017-03-14 : If Arachnus Web enters the battlefield attached to an attacking or blocking creature (due to Arachnus Spinner’s ability, for example), that creature will continue to attack or block.

2017-03-14 : If Arachnus Web’s last ability triggers, but the enchanted creature’s power is reduced to 3 or less before the ability resolves, the ability will have no effect. Arachnus Web won’t be destroyed.

2017-03-14 : If the enchanted creature’s power isn’t 4 or greater when the end step begins, the last ability won’t trigger at all.

2017-03-14 : Triggered abilities (starting with “when,” “whenever,” or “at”) are unaffected.