Multiple ID usage

[bitsnaps]

Hi,
There are two declarations of id="app" in this project, the first one here:

Touring-Vue-Router/src/App.vue

Line 2 in a16e6e1

<div id="app">

and the second one here:

Touring-Vue-Router/public/index.html

Line 14 in a16e6e1

<div id="app"></div>

In one of the real-world-vue project there is only one usage in index.html page.
Is that a mistake? How does vue deal with that?