Also, transclusion never really took off, so traditional web pages are different to an app's page state, which might be initialized from computation by composed

Also, transclusion never really took off, so traditional web pages are different to an app's page state, which might be initialized from computation by composed modules on the backend and a series of asynchronous updates in frontend. CSS has the cascade and @import and background-image links for example, but historically the document was typically one text file and some pictures; an app's UI is combined from many source files and subordinate resources. CSS works pretty well for declaratively styling a monolithic document, but it is not entirely satisfactory for styling the UI of complex app such as Facebook.