padding-inline-start isn’t always horizontal, if writing-mode is writing-mode value changed to vertical-rl or vertical-lr, then the text will be vertical.
block axis is the flow of the website in English/Arabic top to bottom, but in Japanese is right to left, It’s the flow of the website, not the text direction!
Inline axis is the flow of the text and it always be the opposite from the block axis. Example padding-inline-start(text-direction) will be:
in English —padding-left (horizontal)
in Arabic — padding-right(horizontal)
but in Japanese — padding-top(vertical)
I hope it make better sense to you now :-)