Treat custom elements as labelable elements

This change causes the checker to consider all custom elements to be
“labelable elements” per the definition of that term in the spec at
https://html.spec.whatwg.org/multipage/forms.html#category-label.

Without this change, the checker fails to recognize “form-associated
custom elements” with valid id attributes as labelable elements, and so
ends up reporting false positives for any “label” elements in a document
that reference the id attribute of a custom element.

This change isn’t completely conforming to the spec because it
effectively doesn’t distinguish between custom elements that aren’t
“form-associated custom elements” defined in the spec at
https://html.spec.whatwg.org/custom-elements.html#form-associated-custom-element
and custom elements that are true “form-associated custom elements”.

The problem with trying to conform to that spec requirement to only
treat form-associated custom elements as labelable — but not other
custom elements — is that it necessitates executing the document’s
JavaScript and using DOM APIs to examine the document. But in the
checker architecture, that’s not possible — the checker doesn’t
execute JavaScript and can’t use DOM APIs to inspect documents.

Fixes #963

CHANGELOG.md

@@ -10,6 +10,7 @@ NN XXX NNNN
   - Allow `height` and `width` attributes for SVG `symbol` element
   - Allow `capture` attribute for the `input[type=file]` element
   - Allow `disabled` attribute for the `link[rel=stylesheet]` element
+  - Treat custom elements as labelable elements for `label[for]` handling
   - ARIA: Fix bug that disallowed implicit combobox/listbox for `select`
   - ARIA: Improve error message for `alt`-less `img` with ARIA attributes
 

src/nu/validator/checker/schematronequiv/Assertions.java

@@ -1711,6 +1711,7 @@ public void startElement(String uri, String localName, String name,
             boolean languageJavaScript = false;
             boolean typeNotTextJavaScript = false;
             boolean hasAriaAttributesOtherThanAriaHidden = false;
+            boolean isCustomElement = false;
             String xmlLang = null;
             String lang = null;
             String id = null;
@@ -1930,6 +1931,30 @@ public void startElement(String uri, String localName, String name,
                 }
             }
 
+            if (localName.contains("-")) {
+                isCustomElement = true;
+                if (atts.getIndex("", "is") > -1) {
+                    err("Autonomous custom elements must not specify the"
+                            + " \u201cis\u201d attribute.");
+                }
+                try {
+                    CustomElementName.THE_INSTANCE.checkValid(localName);
+                } catch (DatatypeException e) {
+                    try {
+                        if (getErrorHandler() != null) {
+                            String msg = e.getMessage();
+                            if (e instanceof Html5DatatypeException) {
+                                msg = msg.substring(msg.indexOf(": ") + 2);
+                            }
+                            VnuBadElementNameException ex = new VnuBadElementNameException(
+                                    localName, uri, msg, getDocumentLocator(),
+                                    CustomElementName.class, false);
+                            getErrorHandler().error(ex);
+                        }
+                    } catch (ClassNotFoundException ce) {
+                    }
+                }
+            }
             if ("input".equals(localName)) {
                 if (atts.getIndex("", "name") > -1
                         && "isindex".equals(atts.getValue("", "name"))) {
@@ -2661,7 +2686,8 @@ else if ("bdo" == localName && atts.getIndex("", "dir") < 0) {
                     || "output" == localName //
                     || "progress" == localName //
                     || "select" == localName //
-                    || "textarea" == localName) {
+                    || "textarea" == localName //
+                    || isCustomElement) {
                 formControlIds.addAll(ids);
             }
 
@@ -3249,29 +3275,6 @@ else if ("bdo" == localName && atts.getIndex("", "dir") < 0) {
                         // ARIA
                 }
             }
-            if (localName.contains("-")) {
-                if (atts.getIndex("", "is") > -1) {
-                    err("Autonomous custom elements must not specify the"
-                            + " \u201cis\u201d attribute.");
-                }
-                try {
-                    CustomElementName.THE_INSTANCE.checkValid(localName);
-                } catch (DatatypeException e) {
-                    try {
-                        if (getErrorHandler() != null) {
-                            String msg = e.getMessage();
-                            if (e instanceof Html5DatatypeException) {
-                                msg = msg.substring(msg.indexOf(": ") + 2);
-                            }
-                            VnuBadElementNameException ex = new VnuBadElementNameException(
-                                    localName, uri, msg, getDocumentLocator(),
-                                    CustomElementName.class, false);
-                            getErrorHandler().error(ex);
-                        }
-                    } catch (ClassNotFoundException ce) {
-                    }
-                }
-            }
         } else if ("http://n.validator.nu/custom-elements/" == uri) {
             /*
              * For elements with names containing "-" (custom elements), the