APIs needed by Vue

Not a comprehensive list. As of writing, just the ones I encountered when trying to get "hello world" example to work on Vue 2.6.5 (dev).

### window.navigator.userAgent

```
Uncaught TypeError: Cannot read property 'userAgent' of undefined
```

```js
var UA = inBrowser && window.navigator.userAgent.toLowerCase();
```

### document.createEvent()

```
Uncaught TypeError: document.createEvent is not a function
```

```js
// Determine what event timestamp the browser is using. Annoyingly, the
// timestamp can either be hi-res (relative to page load) or low-res
// (relative to UNIX epoch), so in order to compare time we have to use the
// same timestamp type when saving the flush timestamp.
if (inBrowser && getNow() > document.createEvent('Event').timeStamp) {
```

### Set innerHTML

```
Uncaught TypeError: Cannot set property innerHTML of # which has only a getter
```

```js
function getShouldDecode(href) {
div = div || document.createElement('div');
div.innerHTML = href ? "" : "

Start by reproducing the Vue 2.6.5 development hello-world example and review the workerDOM APIs involved in window.navigator.userAgent, document.createEvent(), innerHTML, and the Vue global. Done means the reported TypeErrors and ReferenceError no longer occur for this example, without relying on the suggested workaround.

Written by the indexing model from the issue text.