checkboxes inside select dropdown

is it possible using html/css? and if not, what technologies do you think it might be possible in.

I want a drop-down list box with checkboxes. so instead of just being able to multi-select - there would be checkboxes to multi-select.

I have created this in desktop apps (c++) but I have not seen this, nor aware if it is even possible with web apps.

You could create the Drop-Down using Layers & CSS.
You can them put form objects inside the layers.

You might be able to style something that looks like a select and put check boxes in it in IE but it is a deadend in Netscrap 4. you have to use layers and as soon as you put form elements in layrers it breaks the form.