[css-forms-1] Add 24px sizing rule for checkbox and radio inputs (#12489)

Author: lukewarlow

css-forms-1/Overview.bs

@@ -871,8 +871,12 @@ ISSUE: Refine meter, progress, switch and range input styling.
 
 ```css
 input:is([type=checkbox]:not([switch]), [type=radio]) {
-    width: 1em;
-    height: 1em;
+    /* These size rules ensure accessibility by following WCAG rules:
+     * https://www.w3.org/WAI/WCAG22/Understanding/target-size-minimum.html */
+    block-size: 1em;
+    inline-size: 1em;
+    font-size: max(1em, 24px);
+
     display: inline-flex;
     align-items: center;
     justify-content: center;