In the last few days I have had some opportunities to reflect and compare the frontend architecture of a SPA developed using custom elements with the elements of a microservices backend architecture.
Here are some points of comparison.


Custom element can be seen as microservices (micro frontend) if they are developed with an isolated logic.
However, the structure of the web pages being tree-like and made up of numerous elements, one could conceive that different instances of the same microservice operate for the use of different sub-domains of the same application.


CSS selectors can be considered the equivalent of a DNS system.
CSS selectors allows to refer to an element abstractly from their real memory location, similar to how domain names relate to a network address.

The obtained DOM nodes are equivalent to the addresses/port for the reference of the service.
Also due to the tree nature, the sub-elements can be understood both as services and as the single doors of a parent element depending on the level of dependency they have between them.

Direct communication

Custom element can expose methods that act as synchronous APIs to the service of other elements, similarly to an RPC communication.

Event driven communication

DOM event listeners allow for asynchronous and decoupled communication between different elements.
Container elements, and in particular window and document, can act as pub/sub communication brokers.
Event types are comparable to channels/topics.
Individual elements can emit events of a certain type on the global broker and others subscribe to them.

Environment Variables

Custom element attributes can be thought of as the analog of environment variables or other type of configuration source for the service.


Frontend unit tests can often be seen as integration tests with browser-provided services (webapis), and as with backend integration tests what you can do is use mock interfaces to the service/external API, or implement real communication by running tests on a browser environment equivalent to running test-containers for external services.