Anonymous Answer

I would break it down into 3 categories:
Maintainability:
- Small components, don't make the code take up more than 100 lines
- Folder structure that makes sense.
- Avoid duplicate code, make generic functions to be reused
- Avoid generic variable names but also shouldn't be overly long
- Documentation: comment on things that aren't self-explained by the code and let semantic code take care of the rest
- Style guide: have a general UI/UX style guide and theming within styled components

Scalability:
- Modular components are components that have a single purpose and do that purpose very well and are agnostic to how it's used in the application. But also don't over modularize so that you have too many files interacting with each other
- Choose less opinionated software so you're flexible in how you use it. Like angular is more opinionated, you have to use its built-in router and API fetcher whereas in react you can choose your own and swap them out based on your needs or updates in the web world

Performance:
- Avoid re-renders
- Small bundle size
- Small npm libraries
- Prefetching like for pagination
- Lazy loading like for images.