[css-background-4] Add an issue about syntax for background-position-x and background-position-y.

Author: dbaron

css-backgrounds-4/Overview.bs

@@ -86,6 +86,8 @@ Background Positioning Longhands: the 'background-position-x', 'background-posit
 
 	Issue: This section is still being worked out. The tricky thing is making all the start/end keywords work sanely.
 
+	<p class="issue" id="issue-x-y-val-pct-len">The Value lines below don't allow &lt;percentage&gt; and &lt;length&gt; values to be specified on their own.  This disagrees with existing implementations.
+
 	<pre class='propdef'>
 		Name: background-position-x
 		Value: [ center | [ left | right | x-start | x-end ]? [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]? ]#

css-backgrounds-4/Overview.html

@@ -243,6 +243,7 @@ <h3 class="heading settled" data-level="2.1" id="the-background-position"><span
    <p class="issue" id="issue-f4a1aeb0"><a class="self-link" href="#issue-f4a1aeb0"></a> Specify what happens to set of properties that are not set. Maybe they’re just not set?</p>
    <h4 class="heading settled" data-level="2.1.1" id="background-position-longhands"><span class="secno">2.1.1. </span><span class="content"> Background Positioning Longhands: the <a class="property" data-link-type="propdesc" href="#propdef-background-position-x" id="ref-for-propdef-background-position-x-2">background-position-x</a>, <a class="property" data-link-type="propdesc" href="#propdef-background-position-y" id="ref-for-propdef-background-position-y-2">background-position-y</a>, <a class="property" data-link-type="propdesc" href="#propdef-background-position-inline" id="ref-for-propdef-background-position-inline-2">background-position-inline</a>, and <a class="property" data-link-type="propdesc" href="#propdef-background-position-block" id="ref-for-propdef-background-position-block-2">background-position-block</a> properties</span><a class="self-link" href="#background-position-longhands"></a></h4>
    <p class="issue" id="issue-67389536"><a class="self-link" href="#issue-67389536"></a> This section is still being worked out. The tricky thing is making all the start/end keywords work sanely.</p>
+   <p class="issue" id="issue-x-y-val-pct-len"><a class="self-link" href="#issue-x-y-val-pct-len"></a>The Value lines below don’t allow &lt;percentage> and &lt;length> values to be specified on their own.  This disagrees with existing implementations. </p>
    <table class="def propdef" data-link-for-hint="background-position-x">
     <tbody>
      <tr>
@@ -1271,6 +1272,7 @@ <h2 class="no-num no-ref heading settled" id="issues-index"><span class="content
    <div class="issue"> Specify the value assignment in more detail. Should expand just like Level 3.<a href="#issue-e752f96e"> ↵ </a></div>
    <div class="issue"> Specify what happens to set of properties that are not set. Maybe they’re just not set?<a href="#issue-f4a1aeb0"> ↵ </a></div>
    <div class="issue"> This section is still being worked out. The tricky thing is making all the start/end keywords work sanely.<a href="#issue-67389536"> ↵ </a></div>
+   <div class="issue">The Value lines below don’t allow &lt;percentage> and &lt;length> values to be specified on their own.  This disagrees with existing implementations. <a href="#issue-x-y-val-pct-len"> ↵ </a></div>
    <div class="issue"> For <a class="css" data-link-type="maybe" href="#valdef-corner-shape-scoop">scoop</a> and <a class="css" data-link-type="maybe" href="#valdef-corner-shape-notch">notch</a>, how do the color/style/width transitions work?
 		Do they transition once in the middle, or alternate to match the topness/sideness of that segment of the border? <a href="#issue-7e3f29b5"> ↵ </a></div>
    <div class="issue"> Add a <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-transitions-1/#funcdef-transition-timing-function-cubic-bezier">cubic-bezier()</a> function for random other shapes?