General antonyms:
* Lenient
* Lax
* Flexible
* Permissive
* Tolerant
* Easygoing
* Loose
* Indulgent
* Forgiving
More specific antonyms, depending on context:
* Informal (if referring to rules)
* Casual (if referring to behavior)
* Loose (if referring to interpretation)
* Open-minded (if referring to attitude)
The best antonym depends on the specific context in which you're using the word "strict."