98.css – design system for building faithful recreations of Windows 98 UIs

I don't think there's anything official or even standard for naming. Some places refer to them as class-less:

https://github.com/troxler/awesome-css-frameworks#class-less

And these seem to be the strictly frameworks without classes.

This site refers to them as "drop-in" frameworks (and lets you test them out live):

https://dohliam.github.io/dropin-minimal-css/

The ones here appear to not be strictly class-less, since I know at least some of those frameworks do have classes, but are probably optional/very light on classes. 98.css would also be a "drop in" in this case since you can see things like the title bar do use classes:

    <div class="title-bar">
      <div class="title-bar-text">A Title Bar</div>
      <div class="title-bar-controls">
        <button aria-label="Close"></button>
      </div>
    </div>