[css-text] Correct test to match spec

See #17599
diff --git a/css/css-text/text-align/text-align-last-empty-inline.html b/css/css-text/text-align/text-align-last-empty-inline.html
index 07dcb3b..9bf3681 100644
--- a/css/css-text/text-align/text-align-last-empty-inline.html
+++ b/css/css-text/text-align/text-align-last-empty-inline.html
@@ -13,10 +13,13 @@
     padding: 1px;
     background: black;
   }
+  #reference {
+    text-align: center;
+  }
 </style>
 <body>
-  <div id="justify"><span></span></div>
-  <div id="reference"><span></span></div>
+  <div id="justify"><span>a</span></div>
+  <div id="reference"><span>a</span></div>
   <p>
     The two black line segments above should align.
   </p>
@@ -29,5 +32,5 @@
   const ref_rect = ref_element.firstElementChild.getBoundingClientRect();
   assert_equals(justify_rect.left, ref_rect.left);
   assert_equals(justify_rect.right, ref_rect.right);
-}, 'Left and right edges of empty inlines should align.');
+}, 'content that cannot be justified should be centered when text-align-last is justify');
 </script>