Adds a defintion for essential text change

_rules/auto-update-text-efbfc7.md

@@ -3,7 +3,7 @@ id: efbfc7
 name: Text content that changes automatically can be paused, stopped or hidden
 rule_type: atomic
 description: |
-  This rule checks that for any text content that regularly changes automatically, there are instruments to pause, stop, or hide it or to control its changing frequency.
+  This rule checks that for any non-essential text content that regularly changes automatically, there are instruments to pause, stop, or hide it or to control its changing frequency.
 accessibility_requirements: # Remove whatever is not applicable
   wcag20:2.2.2: # Pause, Stop, Hide (A)
     forConformance: true
@@ -34,7 +34,7 @@ This rule applies to any [HTML element][] that has a [visible][] [text node][] a
 
 ## Expectation
 
-For each test target there is at least one set of [instruments][instrument], where each [instrument][] is in the same [web page][] as the test target or can be found in a [clearly labeled location][] from that [web page][], to achieve at least one of the following objectives:
+For each test target that is not an [essential text change][] there is at least one set of [instruments][instrument], where each [instrument][] is in the same [web page][] as the test target or can be found in a [clearly labeled location][] from that [web page][], to achieve at least one of the following objectives:
 
 - pause and resume the change of the [visible text content][]; or
 - stop the change of the [visible text content][]; or
@@ -45,7 +45,6 @@ For each test target there is at least one set of [instruments][instrument], whe
 
 ## Assumptions
 
-- The auto-updating of the content is not [essential][], which is listed as valid exception to [Success Criterion 2.2.2: Pause, Stop, Hide][sc 2.2.2]. When the auto-updating of content is [essential][] this rule may produce incorrect results.
 - The content being changed automatically is information. If the automatically changing content is not information (for example, an ASCII rendered spinning icon that does not provide information on what time is left for a process to end or how much progress has been made) the rule might fail but the success criterion might still be satisfied.
 - Any [content][] changes are enabled by the content of the [HTML document][] the test target belongs to. Changes originating from any other sources (e.g. browser shortcuts, browser extensions, browser settings, user agents, external browser applications) are not considered.
 - All user actions are transmitted by the user agent to the [HTML document][]. If there are other event sources that result from a user action this rule might fail but the success criterion might still be satisfied.

pages/glossary/essential-text-change.md

@@ -0,0 +1,15 @@
+---
+title: Essential Text Change
+key: essential-text-change
+unambiguous: true
+objective: false
+input_aspects:
+  - Accessibility tree
+  - CSS styling
+  - DOM tree
+---
+
+An _essential text change_ occurs when the modification of the text content of an HTML element is necessary to convey accurate and timely information to the user. An _essential text change_ may appear in any of the following forms:
+
+1. Providing context - Text is changed to provide users with additional information required to understand the current state of the page. For example, while searching for results, text may change to convey the page is *loading* to provide context to the user.
+2. Live updates of important information - The content automatically updates to show the most recent information available from a data source and should not be ignored by the user. For example, updates to a users financial information or browsing sessions status would constitute an `essential-text-change`.