It’s the correct term from the technical vocabulary of book layout and printing. A rule is a line separating things on a page, while a border is a box.[0]. While the term originates in book printing shops, native English speakers would recognize it most easily from the terms “standard ruled” and “college ruled” as applied to loose-leaf notebook paper, describing the faint blue, printed horizontal lines providing guidance for straight handwriting.[1]
I remember this argument from the 90s where large numbers of people were upset at having to learn the world "Font". What it came down to was, either you use the correct existing term, or you create a new term and end up with constant conflict between the two.
Funnily the word "font" is in fact now used in digital publishing for what is more properly called "typeface". In traditional typesetting "font" refers to one specific size, weight and style of a typeface. That is, Helvetica is a typeface, Helvetica Light Oblique 12pt is a font.
While that may be true, I’ll say that as a native English speaker that reading is not the most obvious reading of “row rule”, which to me would be “A determinate method prescribed for performing any operation and producing a certain result.”
Indeed a “CSS rule” is already a thing and it has nothing to do with lines.
While that is right, “divider” or “divider line” are common as well, and easier to search for. Try a search for “rule page layout” or “rule typography” or “rule css”.
Admittedly, we already have <hr> and <table rules="…">.
