Great!!! was searching this only, to get the final output
Btw, is there any way to access the Output Stream from filterContext

Your code has a problem for a little bit hardly scenario, as i.e. replace links...
Using Encoding.UTF8.GetString with a parts of bytes (buffer) is dangerous. For example, a block of bytes may be interrupted in the middle of the html tag...
My suggestion is to collect the list of byte arrays in the Write method, then in Close() combine their into a single whole and process it already...

Very helpful, thanks for sharing!

You saved my Week!
Thanks a lot!