Allow backwards selection (#87)

As discussed in #81, the fact that EditContext prohibits a backwards selection is a source of added complexity for developers.

So as resolved in the Editing WG, remove that restriction.

Closes #81.

index.html

@@ -108,7 +108,7 @@ <h4>EditContext state</h4>
  <ul>
  <li><dfn>text</dfn> which is a {{DOMString}} representing editable content. The initial value is the empty string.</li>
  <li><dfn>selection start</dfn> which refers to the offset in [=text=] where the selection starts. The initial value is 0.</li>
- <li><dfn>selection end</dfn> which refers to the offset in [=text=] where the selection ends. The initial value is 0. [=selection end=] must always be greater than or equal to [=selection start=].</li>
+ <li><dfn>selection end</dfn> which refers to the offset in [=text=] where the selection ends. The initial value is 0. [=selection end=] may be less than [=selection start=] in the case of a "backwards" selection (in reverse of document order).</li>
  <li><dfn>is composing</dfn> which indicates if there is an active composition. The initial value is false.</li>
  <li><dfn>composition start</dfn> which refers to the offset in [=text=] representing the start position of the text being actively composed. The initial value is 0.</p></li>
  <li><dfn>composition end</dfn> which refers to the offset in [=text=] representing the end position of the text being actively composed. The initial value is 0. [=composition end=] must always be greater than or equal to [=composition start=].</li>
@@ -1056,9 +1056,6 @@ <h3 id="editcontext-interface">EditContext Interface</h3>
  <dd>None</dd>
  </dl>
  <ol>
- <li>
- If |start| > |end|, abort these steps.
- </li>
  <li>
  set [=selection start=] to |start|
  </li>