In terms of style isolation, the answer is very much “it depends”. And it depends as much on the nature of what the component does, as the kind of isolation you want to achieve.
- Namespace isolation. Example: you have different components in the same codebase or otherwise meant to work together; you may want assurance that a locally defined style attached to class “foo” doesn’t have unexpected effects on other components which happen to use the same class a different way. This is commonly achieved with build tooling, eg by mangling class names.
- Cascade isolation. Example: you have an embeddable widget that you want to look consistent in any context, regardless of the styles of its parent DOM. This is achievable to some extent without custom elements, but they are a way to achieve it with confidence in a relatively straightforward way (at the expense of other limitations and complexity).
- Namespace isolation. Example: you have different components in the same codebase or otherwise meant to work together; you may want assurance that a locally defined style attached to class “foo” doesn’t have unexpected effects on other components which happen to use the same class a different way. This is commonly achieved with build tooling, eg by mangling class names.
- Cascade isolation. Example: you have an embeddable widget that you want to look consistent in any context, regardless of the styles of its parent DOM. This is achievable to some extent without custom elements, but they are a way to achieve it with confidence in a relatively straightforward way (at the expense of other limitations and complexity).