Short answer: Periods are the default counter suffix for ordered list implementation in web browsers.
Long answer:
If you are using ordered lists in web pages, browses are implementing them using period after. If not specified, the default value will be "\2E\20" (". ", a full stop followed by a space).
You can check a specification example: https://html.spec.whatwg.org/multipage/grouping-content.html#attr-ol-type
The Arabic numerals is just a type from possible predefined counter styles the https://drafts.csswg.org/css-counter-styles-3/#predefined-counters. Because of typography and internationalisation demands, the W3C invented ways to overwrite the defaults.
The suffix for the counter can be changed from the default period to <string>, <image>, or <custom-ident>. Also common I've seen parentheses (")", closed round bracket, called unpaired parenthesis). Depending on the hierarchy of your ordered lists you might need to go multiple levels down, so you might need multiple counter suffix styles (periods for level 1, single parentheses for level 2, paired parentheses for level 3, etc. ).
1.
1.1 ...
(a) ...
2) or (2) ...
3) or (3) ...
Regarding your question about international convention. I don't know, but you can read more about the topic at https://writing.stackexchange.com/a/5685