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Every page load flashes unstyled for a split second before the shadow root finishes attaching.
What you'll have at the end
A server-rendered template tag with shadowrootmode set, so the shadow root and its styles exist the instant the parser reaches it
You need
A shadow-DOM custom element whose class currently builds its shadow root in JavaScript, in the constructor or connectedCallback, served through a pipeline (a static site build step, a server template, or a script that renders the page) able to also emit that same markup as part of the page's first HTML response.
Not covered
Building a server-rendering pipeline from nothing, or hydrating a framework's own reactive state once the page lands: this covers only getting the shadow root's finished markup into the first HTML response and keeping the class from wiping it out once its own script runs.
Package a repeated UI block as a shadow-encapsulated custom element
for building the client-only version of a shadow-DOM component from nothing, before there's any server-rendered markup to move into declarative form
Test a custom element through its public API, not its shadow internals
for turning the by-hand checks in steps 4 and 7 into an automated test instead of confirming them in the console each time
Checked 25 Aug 2026
Part of the Web Components cookbook