Extended HTML Parsing
Custom HTML-like languages can modify the lexing of standard HTML by providing an alternative HTML lexer to the constructor of HtmlCompoundLexer (typically a new instance of HtmlLexerFactory is passed).
However, it is not currently possible to extend the lexing/parsing of standard HTML files, or the standard HTML portions of custom files such as WebForms or Razor, from third party code. An example of this would be to add support for custom templates inside the language, such as Angular's interpolation expressions:
This kind of syntax can be supported by third party code using existing extension mechanisms. For example, if the syntax is simple enough, this could be implemented by References and Daemon support for highlighters. If the expression is more complex, it can also be implemented by using Injected PSI, although this is not very efficient, as the content of the nodes are converted to and from strings on modification. There are also limitations on nested injections. It is best to only use injected PSI for small ranges.
Angular expressions
ReSharper's support for Angular expressions is currently implemented differently, to work around these limitations. The implementation of HtmlProjectFileLanguageService returns HtmlLanguage by default, but returns Angular2HtmlLanguage when Angular support is enabled.
This means that HTML files are treated as "Angular HTML" files, and are now associated with the Angular2HtmlLanguage PSI language type. It also means that the per-language components for HTML files are resolved using Angular2HtmlLanguage instead of HtmlLanguage. One implication of this is that the lexer factory registered for HTML files becomes Angular2HtmlLexerFactory instead of HtmlLexerFactory. This new lexer factory creates a lexer that is derived from HtmlRawLexerGenerated, and parses standard HTML, but also handles expressions that are wrapped in braces.
This does not affect compound lexers - typically, they pass in a new instance of HtmlLexerFactory. To get Angular support in a custom HTML-like language, pass in an instance of Angular2HtmlLexerFactory.
In order to match ReSharper behaviour, the custom language should check to see if Angular support is enabled, by taking in an instance of the AngularIsEnabledProvider shell component, and comparing the GlobalAngularLevel to AngularLevel.None.