The HTML has no way to specify the value to a list item.
The "value" attribute of the HTML "li" element does exactly this, but setting the value isn't really what you need for what GP was asking for; setting the kind of marker is. This can, as you note, be done through CSS, but it can also be done through HTML (using the "type" attribute of the "ol" element.)
In the context of conversion of legal documents, where the marker type is intended for reference, it would be semantically correct to do this in HTML, so it would probably be desirable for markdown to support all the marker types supported by HTML for ordered list as markers for ordered lists, and set the type attribute correctly.
So, using a number should result in the current behavior, using a lowercase letter (other than one which might be a lowercase roman numeral, like "i"!) should result in lowercase-alpha markers, etc.
Comments
The "value" attribute of the HTML "li" element does exactly this, but setting the value isn't really what you need for what GP was asking for; setting the kind of marker is. This can, as you note, be done through CSS, but it can also be done through HTML (using the "type" attribute of the "ol" element.)
In the context of conversion of legal documents, where the marker type is intended for reference, it would be semantically correct to do this in HTML, so it would probably be desirable for markdown to support all the marker types supported by HTML for ordered list as markers for ordered lists, and set the type attribute correctly.
So, using a number should result in the current behavior, using a lowercase letter (other than one which might be a lowercase roman numeral, like "i"!) should result in lowercase-alpha markers, etc.