Is something like this possible?
.imgbox:hover{ .ui-resizable-se { /*some style */ } }
Or a conceptual equivalent? Basically, only when an elem
No, CSS does not allow nesting. You'd have to write it like this:
.imgbox:hover .ui-resizable-se { /*some style */ }
However, there are various CSS preprocessors available which convert something like this in valid CSS. The most popular ones are LESS and SASS/SCSS.