The standard says that "Leading spaces or tabs are skipped" and that "However, the first line may be preceded by up to three spaces of indentation. Four spaces of indentation is too many" for paragraphs, where a "space" is defined as U+0020, but U+3000 (　) is also taken by commonmark.js as valid indentation, in the sense that

　a

converts to

<p>a</p>

on the dingus page.

Also, a link to babelmark for reference: https://babelmark.github.io/?text=%E3%80%80a%0A.