diff --git a/flow-server/src/main/java/com/vaadin/flow/dom/ElementUtil.java b/flow-server/src/main/java/com/vaadin/flow/dom/ElementUtil.java
index 66ab9edcba9..8a45f14b548 100644
--- a/flow-server/src/main/java/com/vaadin/flow/dom/ElementUtil.java
+++ b/flow-server/src/main/java/com/vaadin/flow/dom/ElementUtil.java
@@ -41,7 +41,8 @@ public class ElementUtil {
      * https://www.w3.org/TR/html-markup/syntax.html#tag-name "HTML elements all
      * have names that only use characters in the range 0–9, a–z, and A–Z."
      */
-    private static Pattern tagNamePattern = Pattern.compile("^[a-zA-Z0-9-]+$");
+    private static Pattern tagNamePattern = Pattern
+            .compile("^[a-zA-Z][a-zA-Z0-9-_\\.]*$");
 
     private ElementUtil() {
         // Util methods only
diff --git a/flow-server/src/test/java/com/vaadin/flow/dom/ElementUtilTest.java b/flow-server/src/test/java/com/vaadin/flow/dom/ElementUtilTest.java
index c15add420ef..f929fe30fcb 100644
--- a/flow-server/src/test/java/com/vaadin/flow/dom/ElementUtilTest.java
+++ b/flow-server/src/test/java/com/vaadin/flow/dom/ElementUtilTest.java
@@ -123,4 +123,25 @@ public void toAndFromJsoup() {
                 EXPECTED_TEXT_2,
                 recreatedElement.getChild(0).getChild(1).getText());
     }
+
+    @Test
+    public void isValidTagName_validTagNames() {
+        Assert.assertTrue(ElementUtil.isValidTagName("foo"));
+        Assert.assertTrue(ElementUtil.isValidTagName("foo-bar"));
+        Assert.assertTrue(ElementUtil.isValidTagName("foo_bar"));
+        Assert.assertTrue(ElementUtil.isValidTagName("foo_bar-baz"));
+        Assert.assertTrue(ElementUtil.isValidTagName("foo12.bar3"));
+        Assert.assertTrue(ElementUtil.isValidTagName("foo-._"));
+        Assert.assertTrue(ElementUtil.isValidTagName("x"));
+    }
+
+    @Test
+    public void isValidTagName_invalidTagNames() {
+        Assert.assertFalse(ElementUtil.isValidTagName("1foo"));
+        Assert.assertFalse(ElementUtil.isValidTagName("-foo"));
+        Assert.assertFalse(ElementUtil.isValidTagName("_foo"));
+        Assert.assertFalse(ElementUtil.isValidTagName(".foo"));
+        Assert.assertFalse(ElementUtil.isValidTagName("foo>"));
+        Assert.assertFalse(ElementUtil.isValidTagName("foo$bar"));
+    }
 }