add a lot more parseDOM tests to all extensions

docs/src/demos/Extensions/Underline/index.spec.js

@@ -10,6 +10,20 @@ context('/api/extensions/underline', () => {
     })
   })
 
+  it('should parse u tags correctly', () => {
+    cy.get('.ProseMirror').then(([{ editor }]) => {
+      editor.setContent('<p><u>Example Text</u></p>')
+      expect(editor.html()).to.eq('<p><u>Example Text</u></p>')
+    })
+  })
+
+  it('should transform any tag with text decoration underline to u tags', () => {
+    cy.get('.ProseMirror').then(([{ editor }]) => {
+      editor.setContent('<p><span style="text-decoration: underline">Example Text</span></p>')
+      expect(editor.html()).to.eq('<p><u>Example Text</u></p>')
+    })
+  })
+
   it('the button should underline the selected text', () => {
     cy.get('.demo__preview button:first')
       .click()