Can anyone explain me why `<ul>` elementa cannot be empty?

And why this HTML:

```
<ul>
    <li class="header">
        <a href="#">Work</a>
        <ul/>
    </li>
    <li class="header">
        <a href="#">New</a>
        <ul/>
    </li>
    <li class="header">
        <a href="#">Ungrouped</a>
        <ul/>
    </li>
    <li class="header">
        <a href="#">Offline</a>
        <ul/>
    </li>
</ul>

```

is rendered like this:

> 
> * Work
> 	+ New
> 		- Ungrouped
> 			* Offline
> 
> 
>