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

sky/specs/markup.md

 Sky Markup: Syntax
 ==================
 
 A Sky file must consist of the following components:
 
  1. If the file is intended to be a top-level Sky application, the
     string "``#!mojo mojo:sky``" followed by a U+0020, U+000A or
     U+000D character.
 
     If the file is intended to be a module, then the string "SKY", a
@@ skipping 126 matching lines @@
         3. Same sequence of characters as "tag name" above.
         4. ``>``
 
 
 Sky Markup: Elements
 ====================
 
 The Sky language consists of very few elements, since it is expected
 that everything of note would be provided by frameworks.
 
+The following elements are implicitly registered by default, even if
+you haven't imported anything. You can get to their constructors if
+you import sky:core (basically, sky:core is always imported by defaul;
+it's the runtime library). None of these elements have shadow trees.
+
 ``<import src="foo.sky">``
  - Downloads and imports foo.sky in the background.
 
 ``<import src="foo.sky" as="foo">``
  - Downloads and imports foo.sky in the background, using "foo" as its
    local name (see ``<script>``).
 
 ``<template>``
  - The contents of the element aren't placed in the Element itself.
    They are instead placed into a DocumentFragment that you can obtain
@@ skipping 12 matching lines @@
 ``<content>``
 ``<content select="...">``
  - In a shadow tree, acts as an insertion point for distributed nodes.
    The select="" attribute gives the selector to use to pick the nodes
    to place in this insertion point; it defaults to everything.
 
 ``<img src="foo.bin">``
  - Sky fetches the bits for foo.bin, looks for a decoder for those
    bits, and renders the bits that the decoder returns.
 
+``<div>``
+ - Element that does nothing.
+
+``<span>``
+ - Element that does nothing.
+
 ``<iframe src="foo.bin">``
  - Sky tells mojo to open an application for foo.bin, and hands that
    application a view so that the application can render appropriately.
 
 ``<t>``
  - Within a ``<t>`` section, whitespace is not trimmed from the start and
    end of text nodes by the parser.
    TOOD(ianh): figure out if the authoring aesthetics of this are ok
 
 ``<a href="foo.bin">``
  - A widget that, when invoked, causes mojo to open a new application
    for "foo.bin".
 
 ``<title>``
  - Sets the contents as the document's title (as provided by Sky to
    the view manager). (Actually just ensures that any time the element
    is mutated, theTitleElement.ownerScope.ownerDocument.title is set
    to the element's contents.)
 
+``<error>``
+ - Represents a parse error.
+
 
 Sky Markup: Global Attributes
 =============================
 
 The following attributes are available on all elements:
 
 * ``id=""`` (any value)
 * ``class=""`` (any value, space-separated)
 * ``style=""`` (declaration part of a Sky style rule)
 * ``lang=""`` (language code)
 * ``dir=""`` (ltr or rtl only)
 * ``contenteditable=""`` (subject to future developments)
 * ``tabindex=""`` (subject to future developments)