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vue/packages/vue-server-renderer at dev · vuejs/vue ( (2017-03-23 12:29:05 +09:00)) https://github.com/vuejs/vue/tree/dev/packages/vue-server-renderer#client-side-hydration

In server-rendered output, the root element will have the server-rendered="true" attribute. On the client, when you mount a Vue instance to an element with this attribute, it will attempt to "hydrate" the existing DOM instead of creating new DOM nodes.