Are online TEFL courses valid?

Based on my experience teaching and living in East Asia I would say that online TEFL certifications are usually accepted when required by the school. The requirements to teach abroad depend on the school and sometimes country. Rarely are TEFL certifications required for any visa application, however certain schools may prefer teachers to have one.

However, just like your degree (or any onsite course) they don't guarantee anything everywhere.

How to check if a TEFL/TESOL certificate is valid?