Back in the day, this probably meant one of a handful of things:
* manually writing all the HTML and putting the inline styles there
* some kind of PHP thingy where you would be essentially rolling your own CSS variables
* some kind of Perl thingy where a verySmart dev is trying to maintain an entire CMS using only regexes
Today, the user could be "having inline styles" in one of 10,000 frameworks-- Vue thingies, React thingies, or perhaps a build flag in a Javascript thingy that spits out a CSS thingy...