Add section on link lifetime (#248)

index.bs

@@ -99,14 +99,31 @@ URLs that direct the user to the answer they are looking for in the page rather
 than linking to the top of the page.
 
 ### User sharing ### {#user-sharing}
-With text fragments, browsers may implement an option to 'Copy URL to here'
+With text directives, browsers may implement an option to 'Copy URL to here'
 when the user opens the context menu on a text selection. The browser can then
 generate a URL with the text selection appropriately specified, and the
 recipient of the URL will have the specified text conveniently indicated.
 Without text fragments, if a user wants to share a passage of text from a page,
 they would likely just copy and paste the passage, in which case the receiver
 loses the context of the page.
 
+## Link Lifetime ## {#link-lifetime}
+
+This specification attempts to maximize the useful lifetime of text directive links, for example, by
+using the actual text content as the URL payload, and allowing a fallback element-id fragment.
+However, pages on the web often update and change their content. As such, links like this may "rot"
+in that the text content they point to no longer exists on the destination page.
+
+Text directive links can be useful despite this problem. In user sharing use cases, the link is
+often transient, intended to be used only within a short time of sending. For longer duration use
+cases, such as references and web page links, text directives are still valuable since they degrade
+gracefully into an ordinary link. Additionally, the presence of a stale text directive can be useful
+information to surface to a user, to help them understand the link creator's original intent and
+that the page content may have changed since the link was created.
+
+See [[#generating-text-fragment-directives]] for best practices on how to create robust text
+directive links.
+
 # Description # {#description}
 
 ## Indication ## {#indication}

index.html

@@ -865,6 +865,7 @@ <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
         <li><a href="#web-text-references"><span class="secno">2.1.1</span> <span class="content">Web text references</span></a>
         <li><a href="#user-sharing"><span class="secno">2.1.2</span> <span class="content">User sharing</span></a>
        </ol>
+      <li><a href="#link-lifetime"><span class="secno">2.2</span> <span class="content">Link Lifetime</span></a>
      </ol>
     <li>
      <a href="#description"><span class="secno">3</span> <span class="content">Description</span></a>
@@ -958,13 +959,26 @@ <h4 class="heading settled" data-level="2.1.1" id="web-text-references"><span cl
 URLs that direct the user to the answer they are looking for in the page rather
 than linking to the top of the page. 
    <h4 class="heading settled" data-level="2.1.2" id="user-sharing"><span class="secno">2.1.2. </span><span class="content">User sharing</span><a class="self-link" href="#user-sharing"></a></h4>
-    With text fragments, browsers may implement an option to 'Copy URL to here'
+    With text directives, browsers may implement an option to 'Copy URL to here'
 when the user opens the context menu on a text selection. The browser can then
 generate a URL with the text selection appropriately specified, and the
 recipient of the URL will have the specified text conveniently indicated.
 Without text fragments, if a user wants to share a passage of text from a page,
 they would likely just copy and paste the passage, in which case the receiver
 loses the context of the page. 
+   <h3 class="heading settled" data-level="2.2" id="link-lifetime"><span class="secno">2.2. </span><span class="content">Link Lifetime</span><a class="self-link" href="#link-lifetime"></a></h3>
+   <p>This specification attempts to maximize the useful lifetime of text directive links, for example, by
+using the actual text content as the URL payload, and allowing a fallback element-id fragment.
+However, pages on the web often update and change their content. As such, links like this may "rot"
+in that the text content they point to no longer exists on the destination page.</p>
+   <p>Text directive links can be useful despite this problem. In user sharing use cases, the link is
+often transient, intended to be used only within a short time of sending. For longer duration use
+cases, such as references and web page links, text directives are still valuable since they degrade
+gracefully into an ordinary link. Additionally, the presence of a stale text directive can be useful
+information to surface to a user, to help them understand the link creator’s original intent and
+that the page content may have changed since the link was created.</p>
+   <p>See <a href="#generating-text-fragment-directives">§ 4 Generating Text Fragment Directives</a> for best practices on how to create robust text
+directive links.</p>
    <h2 class="heading settled" data-level="3" id="description"><span class="secno">3. </span><span class="content">Description</span><a class="self-link" href="#description"></a></h2>
    <h3 class="heading settled" data-level="3.1" id="indication"><span class="secno">3.1. </span><span class="content">Indication</span><a class="self-link" href="#indication"></a></h3>
    <div class="note" role="note">This section is non-normative</div>