Mozilla's web engine is called Gecko and can supposedly be embedded. I say supposedly because, Mozilla rarely ever promote this feature and their Gecko documentation warns readers the documentation is out of date:
https://developer.mozilla.org/en-US/docs/Mozilla/Gecko/Gecko_Embedding_Basics
Given the growing popularity of Electron, despite its memory-heavy and bloated file size, should Mozilla promote embedding Gecko more prominently (assuming it can beat Electron on file size and memory usage)?
What other options are there for embedding a web rendering engine in your app? Are there any lightweight solutions?