Use <template for> and processing instruction ranges (<?marker>, <?start>, and <?end> to update existing parts of the document without JS.
On its own, this provides a performance optimization for cases when the server is slow at producing parts of a document's content that might need to appear early in the DOM. It is part of a bigger initiative (described in the explainer) to enable developers to be able to utilize same-document updates in a way that's built in to the platform.
Explainers: https://github.com/WICG/declarative-partial-updates/blob/main/patching-explainer.md