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.

Microservices⌗

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.

Networking⌗

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.

Tests⌗

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.