Specs: Drop the concept of late-bound elements. An element can never be upgraded.

sky/examples/home.sky

Index: sky/examples/home.sky
diff --git a/sky/examples/home.sky b/sky/examples/home.sky
index 5cfbaecab44bdfbc2d4a1caa25c8e2c68a9d64c1..7580b5eb72a851df588a8060e5dc44de8cd903c0 100644
--- a/sky/examples/home.sky
+++ b/sky/examples/home.sky
@@ -1,9 +1,9 @@
 #!mojo mojo:sky
-<sky>
+<div><!-- remove this element once Document can have multiple element children -->
 <style>
- h1 { font-size: 2em; margin: 1em; }
- p { margin: 0.5em 1em; color: #bcd8f5; font-weight: 900; }
+ .h1 { font-size: 2em; margin: 1em; }
+ .p { margin: 0.5em 1em; color: #bcd8f5; font-weight: 900; }
 </style>
-<h1>about:blank</h1>
-<p>Welcome to Sky!</p>
-</sky>
+<div class="h1">about:blank</div>
+<div class="p">Welcome to Sky!</div>
+</div>