Web based application problem

One thing the internet should do better is the consistency check of the data retrieved.
I know, there are suppose to be Network level check, but what im focusing on is the streamed/downloaded data.
Is it complete? or is it half way done.

One good example is downloading a huge file on the internet.
If its a zip file, its easy. Because we can check the zip file crc from the achieve application.

But isnt this suppose to be a web browser job?

There have been more and more Ajax base application out there, but yet,
i find that its not consistant in terms of opening the processed ajax output.
There are times when the internet got instable , and the browser thought the process is complete.
There are also time when the browser thought the javascript is downloaded complete, but its not.
This lead to instable output or undesired response from the server.
Most of the time, this could be fixed by refreshing, or clearing the cache and refresh.
But i think its best that there should be some kind of browser parity or consistancy check for completion, or dont render the output and output some kind of error message to the user.