Add a cssselect method to all elements, not just HtmlElement

translator defaults to 'xml' in _Element and 'html' in HtmlElement

src/lxml/html/__init__.py

@@ -503,7 +503,8 @@ class HtmlComment(etree.CommentBase, HtmlMixin):
     pass
 
 class HtmlElement(etree.ElementBase, HtmlMixin):
-    pass
+    # Override etree.ElementBase.cssselect, despite the MRO
+    cssselect = HtmlMixin.cssselect
 
 class HtmlProcessingInstruction(etree.PIBase, HtmlMixin):
     pass

src/lxml/lxml.etree.pyx

@@ -1449,6 +1449,20 @@ cdef public class _Element [ type LxmlElementType, object LxmlElement ]:
                                           smart_strings=smart_strings)
         return evaluator(_path, **_variables)
 
+    def cssselect(self, expr, *, translator='xml'):
+        """
+        Run the CSS expression on this element and its children,
+        returning a list of the results.
+
+        Equivalent to lxml.cssselect.CSSSelect(expr)(self) -- note
+        that pre-compiling the expression can provide a substantial
+        speedup.
+        """
+        # Do the import here to make the dependency optional.
+        from lxml.cssselect import CSSSelector
+        return CSSSelector(expr, translator=translator)(self)
+
+
 
 cdef extern from "etree_defs.h":
     # macro call to 't->tp_new()' for fast instantiation