This is true, but the set of custom controls is quite small. Controls that were built-in to, say, Visual Basic 4.0 in 1995 are not present in today's web browse

This is true, but the set of custom controls is quite small. Controls that were built-in to, say, Visual Basic 4.0 in 1995 are not present in today's web browsers. Re-implementing built-in controls (the example of a <select> box is given in the OP) is obviously a very bad idea in almost all circumstances. Implementing a missing control is a much more difficult decision, and there are times when it can be a defensible choice to do so.