Ok, so it might be a long shot, but I would say that 1. the browsers were inconsistent in 1990-2000 so we started using JS to make them behave the same 2. meanwhile the only thing we needed were good CSS styles which were not yet present and consistent behaviour 3. over the years the browsers started behaving the same (mainly because Highlander rules - there can be only one, but Firefox is also coping well) 4. but we…
at the current technology we could cope with server generated old-school web pages because they would have low footprint, work faster and require less memory. unless you have a high latency internet connection: https://news.ycombinator.com/item?id=44326816
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by the way in 2005 I programmed using very funny PHP framework PRADO that was sending every change in the UI to the server. Boy it was slow and server heavy. This was the direction we should have never gone...