my $tree = Continuity::Widget::DomNode->create(
    html => [
      head => [
        style  => { src => '/css/style.css' },
        script => { language => 'javascript', src => '/js/jquery.js' },
        script => { language => 'javascript', src => '/js/app.js' },
      ],
      body => [
        $header->render,
        $content->render,
        $footer->render,
      ]
    ]
  );

So now we have this DOM thingie. Beautiful. What else can we do with it? One
thing is to look at those render methods. Rather than returning a DomNode, lets
say they return a subref. What we'll do is traverse the tree again, and replace
whatever they gave us with the new thing. This is a way for nodes to delay
their rendering until they can get some more context.

So... in $content->render we might have:

  sub render {
    my ($self) = @_;
    return map { my $child = $_; sub { $child->render_on(@_) } } @{$self->children};
  }

  # or

  sub render {
    my ($self) = @_;
    return sub { $self->render_on(@_) };
  }

The body could be re-written directly as:

  body => [
    sub { $header->render_on(@_) },
    sub { $content->render_on(@_) },
    sub { $footer->render_on(@_) },
  ]

Though that wouldn't be as much fun.

A subref could return either a list of DomNodes (often just one) or a list that
can be passed to DomNode->create (including subrefs). The subref will be passed the parent (context) node, or perhaps a dummy node for its exact location.

In render_on we can then use CSS-like thingies to determine some context-specific settings. Lets say we have some magical CSS like:

  .edit EPFarms::User .field {
    mode: edit;
  }

  .edit * {
    mode: edit;
  }

then we have

  sub render_on {
    my ($self, $node) = @_;
    $self->set_context($node);
    if($self->attr('mode') eq 'edit') {
      return (input => { type => 'text', name => $self->htmlname, value => $self->val });
    }
    if($self->attr('mode') eq 'view') {
      return ( $self->val );
    }
  }

So... when does subref expansion happen? It could be explicit, or it could be
implicit as part of to_string for actual sending to the browser:

  $tree->expand_subs;
  $str = $tree->to_string;
  # or
  $str = $tree->to_string;