fix: allow MathML elements in template compiler

Author: nikhil6393

packages/@lwc/engine-server/src/__tests__/fixtures.spec.ts

@@ -101,9 +101,7 @@ async function compileFixture({
                 // IGNORED_SLOT_ATTRIBUTE_IN_CHILD is fine; it is used in some of these tests
                 message.includes('LWC1201') ||
                 message.includes('-h-t-m-l') ||
-                code === 'CIRCULAR_DEPENDENCY' ||
-                // TODO [#5010]: template-compiler -> index -> validateElement generates UNKNOWN_HTML_TAG_IN_TEMPLATE for MathML elements
-                message.includes('LWC1123');
+                code === 'CIRCULAR_DEPENDENCY';
             if (!shouldIgnoreWarning) {
                 throw new Error(message);
             }

packages/@lwc/template-compiler/src/parser/index.ts

@@ -1681,6 +1681,7 @@ function validateElement(
             ast.isBaseLwcElement(element) ||
             KNOWN_HTML_AND_SVG_ELEMENTS.has(tag) ||
             SUPPORTED_SVG_TAGS.has(tag) ||
+            namespace === MATHML_NAMESPACE ||
             DASHED_TAGNAME_ELEMENT_SET.has(tag);
 
         if (!isKnownTag) {