How does html5-script-attribute "data-main" work?
How is that (script load-order) possible?
Because that is the purpose of require.js which consists of rather a lot of code to do that.
That specific part is rather trivial, require.js can't do anything (including load another script) until it is loaded itself.
If the name of the js-file was nomain.js, would the data-attribute be "data-nomain" ?
No. Require looks at data-main to determine the entry point script file. It gets the URL from the value of that attribute.
I see no information about that in the html5 specification or am I looking at wrong place?
data-* attributes are defined in the section 3.2.5.9 Embedding custom non-visible data with the data-* attributes