Add test for get_default_css.

tests/test_dom.py

@@ -45,6 +45,43 @@ def test_validate():
         node.toggle_class("1")
 
 
+def test_get_default_css():
+    class A(DOMNode):
+        pass
+    class B(A):
+        pass
+    class C(B):
+        DEFAULT_CSS = "C"
+    class D(C):
+        pass
+    class E(D):
+        DEFAULT_CSS = "E"
+    node = DOMNode()
+    node_css = node.get_default_css()
+    a = A()
+    a_css = a.get_default_css()
+    b = B()
+    b_css = b.get_default_css()
+    c = C()
+    c_css = c.get_default_css()
+    d = D()
+    d_css = d.get_default_css()
+    e = E()
+    e_css = e.get_default_css()
+
+    # Descendants that don't assign to DEFAULT_CSS don't add new CSS to the stack.
+    assert len(node_css) == len(a_css) == len(b_css) == 0
+    assert len(c_css) == len(d_css) == 1
+    assert len(e_css) == 2
+
+    # Descendants do push the priority of the ancestors' rules down.
+    assert c_css[0][2] == d_css[0][2] + 1 == 0
+
+    # The CSS on the stack is the correct one.
+    assert e_css[0][1:] == ("E", 0)
+    assert e_css[1][1:] == ("C", -2)
+
+
 @pytest.fixture
 def search():
     """