single-spa is a JavaScript metaframework that allows us to build micro frontends using different frameworks that can coexist in a single application. In general, we don't recommend using more than one framework for an application, but there are times when we can't avoid doing so. For instance, single-spa can be quite useful when you're working with a legacy application and you want to experiment by developing a new feature, with either a new version of the existing framework or a completely different one. Given the short life span of many JavaScript frameworks, we see a need for a solution that would allow for future framework changes and localized experimentation, without affecting the entire application. single-spa seems to be a good start in that direction.