> rendering direction should follow reading convention.
What does that even mean in this context? In a strictly LTR language, sure, you read left-to-right and the glyphs are rendered left-to-right. But the whole discussion is about bidirectional text, where the text is rendered by a complex algorithm. What is the “rendering direction”?
I know just enough about some RTL languages to know that one can absolutely intersperse RTL text with, say, and English phrase, and you still read the first (leftmost in the group) English sound first and so on :)
if you write (with a pen) text with mixed arabic, numbers, english words. then somebody else gets to read it, he will read arabic from right to left, then when when he encouters a number or an english word, he will naturally jump to the first latin letter of the word and start reading left to right, then jump back to the beginning if the next arabic word and switch back to RTL. the alogorithm should copy that behaviour.