diff --git a/core/modules/utils/fakedom.js b/core/modules/utils/fakedom.js
index f1f0cacfb..b3611ae2d 100755
--- a/core/modules/utils/fakedom.js
+++ b/core/modules/utils/fakedom.js
@@ -115,7 +115,9 @@ var TW_Element = function(tag, namespace) {
 	this.children = [];
 	this._style = {}; // Internal style object
 	this.style = new TW_Style(this); // Proxy for style management
-	this.namespaceURI = namespace || "http://www.w3.org/1999/xhtml";
+	// createElementNS with empty-string or null normalises to null (no namespace) per spec.
+	// https://dom.spec.whatwg.org/#dom-document-createelementns
+	this.namespaceURI = namespace !== undefined ? (namespace || null) : "http://www.w3.org/1999/xhtml";
 };
 
 
@@ -206,7 +208,16 @@ TW_Element.prototype.addEventListener = function(type,listener,useCapture) {
 
 Object.defineProperty(TW_Element.prototype, "tagName", {
 	get: function() {
-		return this.tag || "";
+		if(!this.tag) {
+			return "";
+		}
+		// HTML elements report uppercase tagName per DOM spec. Other namespaces
+		// preserve case. Fakedom only models HTML documents.
+		// https://dom.spec.whatwg.org/#dom-element-tagname
+		if(this.namespaceURI === "http://www.w3.org/1999/xhtml") {
+			return this.tag.toUpperCase();
+		}
+		return this.tag;
 	}
 });
 
diff --git a/editions/test/tiddlers/tests/test-fakedom.js b/editions/test/tiddlers/tests/test-fakedom.js
index dd5c79793..c58fa8fa7 100644
--- a/editions/test/tiddlers/tests/test-fakedom.js
+++ b/editions/test/tiddlers/tests/test-fakedom.js
@@ -19,6 +19,56 @@ describe("fakedom tests", function() {
 		expect($tw.fakeDocument.createTextNode("text").TEXT_NODE).toBe(3);
 	});
 
+	// Per DOM spec, tagName returns the HTML-uppercased qualified name for HTML
+	// elements. Other namespaces preserve case.
+	// https://dom.spec.whatwg.org/#dom-element-tagname
+	var HTML_NS = "http://www.w3.org/1999/xhtml";
+	var SVG_NS = "http://www.w3.org/2000/svg";
+
+	it("tagName uppercases for HTML namespace", function() {
+		// Default namespace is HTML
+		expect($tw.fakeDocument.createElement("div").tagName).toBe("DIV");
+		// The exact predicate the select widget relies on (#9839)
+		expect($tw.fakeDocument.createElement("optgroup").tagName === "OPTGROUP").toBe(true);
+		// Already-uppercase input stays uppercase
+		expect($tw.fakeDocument.createElement("OPTGROUP").tagName).toBe("OPTGROUP");
+		// Mixed-case input is normalised
+		expect($tw.fakeDocument.createElement("Div").tagName).toBe("DIV");
+		// Hyphenated custom-element names uppercase whole tag, hyphens survive
+		expect($tw.fakeDocument.createElement("my-button").tagName).toBe("MY-BUTTON");
+		// Empty tag returns empty string
+		expect($tw.fakeDocument.createElement("").tagName).toBe("");
+		// Explicit HTML namespace via createElementNS uppercases the same way
+		expect($tw.fakeDocument.createElementNS(HTML_NS,"Div").tagName).toBe("DIV");
+	});
+
+	it("tagName preserves case for non-HTML namespaces", function() {
+		// SVG: lowercase preserved
+		expect($tw.fakeDocument.createElementNS(SVG_NS,"circle").tagName).toBe("circle");
+		// SVG: camelCase preserved (linearGradient is the canonical example)
+		expect($tw.fakeDocument.createElementNS(SVG_NS,"linearGradient").tagName).toBe("linearGradient");
+		// SVG: already-uppercase input is also preserved (NOT lowercased)
+		expect($tw.fakeDocument.createElementNS(SVG_NS,"DIV").tagName).toBe("DIV");
+		// Empty namespace string is "no namespace", not HTML. Case preserved.
+		expect($tw.fakeDocument.createElementNS("","div").tagName).toBe("div");
+	});
+
+	it("tagName reflects current state without mutating it", function() {
+		// Reading tagName must not overwrite the internal `tag` field
+		var el = $tw.fakeDocument.createElement("div");
+		expect(el.tagName).toBe("DIV");
+		expect(el.tag).toBe("div");
+		// Idempotent: two reads return identical values
+		var first = el.tagName, second = el.tagName;
+		expect(first).toBe(second);
+		// Dynamic namespace change is reflected. The getter must read current
+		// state, not a value cached at construction time.
+		var dynamic = $tw.fakeDocument.createElement("foo");
+		expect(dynamic.tagName).toBe("FOO");
+		dynamic.namespaceURI = SVG_NS;
+		expect(dynamic.tagName).toBe("foo");
+	});
+
 	// Real CSSStyleDeclaration returns undefined for Symbol property keys.
 	// Without a guard, the TW_Style Proxy throws on Symbol access. This bites
 	// in practice when Jasmine pretty-prints fakedom elements on failure.
diff --git a/editions/tw5.com/tiddlers/releasenotes/5.5.0/#9843-fakedom-tagname-spec.tid b/editions/tw5.com/tiddlers/releasenotes/5.5.0/#9843-fakedom-tagname-spec.tid
new file mode 100644
index 000000000..33993cae4
--- /dev/null
+++ b/editions/tw5.com/tiddlers/releasenotes/5.5.0/#9843-fakedom-tagname-spec.tid
@@ -0,0 +1,13 @@
+change-category: internal
+change-type: bugfix
+created: 20260711014241000
+description: fakedom tagName is uppercase for HTML elements, matching the DOM spec
+github-contributors: pmario
+github-links: https://github.com/TiddlyWiki/TiddlyWiki5/pull/9843
+release: 5.5.0
+tags: $:/tags/ChangeNote
+title: $:/changenotes/5.5.0/#9843
+type: text/vnd.tiddlywiki
+
+* `$tw.fakeDocument` elements now report `tagName` in uppercase for HTML elements, matching the DOM specification and real browsers; code that compares fakedom `tagName` against lowercase strings must be updated
+* Creating an element with an empty or null namespace now produces a plain element without a namespace