It's hard to know exactly what you mean by "simple", but I'm fairly sure you want to avoid any frameworks, as those tend to have a learning curve, even for the easiest ones. Same goes for build tools like Webpack, which I still have trouble with after years of use.
For CSS I see a lot of recommendations for "classless" libraries, which I've never tried, but could be good if you don't need to do much tinkering. For just a little bit more time investment you can get a lot more options with something like Bootstrap. It's pretty easy to learn, has a lot of "drag and drop" style components, and there are tons of templates out there (free and premium). I learned it once many years ago and have been using it for projects ever since.
On the interactivity side it really depends on what your needs are, but if you don't need too many bells and whistles you could probably get by with vanilla Javascript. jQuery is also a good choice for simple applications, as it offers a slightly more intuitive interface to some of the basic JS actions.
All of this being said, if you think you will be spending a lot of time on the frontend, or have complex interactivity needs, it might be worth it to learn a framework. I can recommend Vue as one that is fairly easy to get started with and also scalable to larger applications.
For CSS I see a lot of recommendations for "classless" libraries, which I've never tried, but could be good if you don't need to do much tinkering. For just a little bit more time investment you can get a lot more options with something like Bootstrap. It's pretty easy to learn, has a lot of "drag and drop" style components, and there are tons of templates out there (free and premium). I learned it once many years ago and have been using it for projects ever since.
On the interactivity side it really depends on what your needs are, but if you don't need too many bells and whistles you could probably get by with vanilla Javascript. jQuery is also a good choice for simple applications, as it offers a slightly more intuitive interface to some of the basic JS actions.
All of this being said, if you think you will be spending a lot of time on the frontend, or have complex interactivity needs, it might be worth it to learn a framework. I can recommend Vue as one that is fairly easy to get started with and also scalable to larger applications.