Not that bizarre. All those other things are called "elements" because the word element just means something like "Essential part" You can have elements of just about anything if they describe the basic building blocks of that thing.
Discussion
This. They are essential components, much like the elements (of the periodic table) are the basic building blocks in chemistry. In a similar fashion, the elements of a web page are the foundational building blocks.
But physical things are not made up of elements, they're made up of atoms, which are instances of elements.
I disagree, elements in all other cases are an abstraction. In html, an element is not a type, it is an instance. It's like looking at a gold coin and calling it "a gold element".
Well, that is a good point. There is probably some defense in calling that gold coin an "element" of a particular payment, in the way that any particular set of tags constitute an element of a particular html document. But yeah, you are right. HTML elements are instances.