They're all from an era where javascript couldn't do everything. Those various crud of "like Gecko" were to get servers to deploy HTML that fit the pattern of

They're all from an era where javascript couldn't do everything. Those various crud of "like Gecko" were to get servers to deploy HTML that fit the pattern of another browser. There were websites that gave you error pages if you weren't Internet Explorer. Nowadays it is far less necessary, but there are still browser-specific stuff involved.