Browsers disagreed about everything

The libraries existed because nothing agreed

Before the DOM APIs converged, the same operation needed different code per browser. That is the whole reason a compatibility layer was worth loading.

Two rendering modes, because the old pages had to keep working

Quirks mode exists so documents written against earlier behaviour would not break when the standards mode arrived. It is still there.

A support table became the thing people argued from

Can I Use turned browser support into a citable artefact, which changed technical arguments from opinion into a shared reference.