The state of the art models these days would not be able to be bundled with browsers. These are typically neural models [0] and require specialized hardware to run, and also have high memory requirements. The models in browsers are light-weight to be able to run on most devices. Might be similar to those used in smartphone keyboards like GBoard [1].

[0] https://www.microsoft.com/en-us/research/blog/speller100-zer..., https://cloud.google.com/blog/products/g-suite/everyday-ai-b...

[1] https://ai.googleblog.com/2021/10/grammar-correction-as-you-...