While on Frontend from the beginning, specially i you are working with a team, you will have to make a lot of small decisions that can cause all kinds of trouble down the line:
- How you build the project, javascript transpilers and all that mess.
- Decide a CSS framework, or not use a framework and then decide on an internal style guide.
- Decide on a component library.
- Decide on how to handle state.
Or you can skip those above and use pure HTML+CSS+Javascript, but those usually end up as very spaguettified and idiosyncratic projects, I rarely see anyone even considering it unless it's a personal project. And for a good reason, the structure provided by component and state handling libraries reduces a lot of the inherent chaos that is structuring frontend web project. But each of those also comes with its own bag of new problems, and complexity that requires the developers to handle a lot of disparate information in their heads while working on a project.