Support url() in content css property
```html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>image-content.html</title>
<style>
@page {
size: 300pt 300pt;
margin: 50pt;
@top-center {
content: "foo " url("icon.png") " bar";
padding: 5px;
background: tomato;
}
}
</style>
</head>
<body>
Hello World
</body>
</html>
```
issue