404.html: better content vertical alignment

Content vertical alignment using `display: table` for `<html>` and `display: table-cell` for `<body>`.
That way one can modify the content and don't worry about readjusting the margin.
I'd also like to suggest adding reasonable line height for `h1` heading since this current 1.5 looks really weird when your content is long enough to span on multiple lines.

```css
h1 {
	line-height: 0.9;
	margin-bottom: 0.3em;
}
```