These methods are useful for verification in tests, e.g.
```js
expect(await page.isEnabled(':text("Remove All")')).toBe(false);
await page.click(':text("Add Item")');
expect(await page.isVisible('.item:text("new item")')).toBe(true);
expect(await page.isEnabled(':text("Remove All")')).toBe(true);
```
When parsing CSS, we assume everything is a valid CSS function,
unless it is in the list of custom functions. This way we'll parse
future CSS functions automatically.
This enables filling the input based on the connected label:
```html
<label for=target>Name</label><input id=target>
```
```js
await page.fill('text=Name', 'Alice');
```
We used to do fetch() to decode the file buffer. However, this is
blocked by strict CSP policy. Instead, we can use explicit
string -> bytes conversion, and trade performance for CSP compliance.
