In the software sense, vanilla means "in it's original form, without addons, modifications, abstractions, third party libraries, ...". Like a "vanilla kernel"

In the software sense, vanilla means "in it's original form, without addons, modifications, abstractions, third party libraries, ...". Like a "vanilla kernel" is a Linux kernel without (distributor) patches. Or a "vanilla debian" is a Debian system without third party repositories or software. Regarding parent, I could say... you cannot make a permanent To-Do web app, with HTML+CSS... drag and drop? data persistence? you need a backend (and JS for event handling) or as in this case local storage via JS. OK, you can make a simple To-Do with static forms being sent to a backend... but then you have HTML+CSS+something (java, python, php, ruby, perl, golang, whatever), not only HTML+CSS.