Only download elements with rel={stylesheet,icon}

Websites may set the rel attribute to whatever they want, so we just
care about stylesheets and icons and not the other stuff like wss
links (looking at you, GitHub), RSS-feeds, ...

qutebrowser/browser/mhtml.py

@@ -119,6 +119,23 @@ def _get_css_imports(data, inline=False):
         return _get_css_imports_cssutils(data, inline)
 
 
+def _check_rel(element):
+    """Return true if the element's rel attribute fits our criteria.
+
+    rel has to contain 'stylesheet' or 'icon'. Also returns True if the rel
+    attribute is unset.
+
+    Args:
+        element: The WebElementWrapper which should be checked.
+    """
+    if 'rel' not in element:
+        return True
+    must_have = {'stylesheet', 'icon'}
+    rels = [rel.lower() for rel in element['rel'].split(' ')]
+    return any(rel in rels for rel in must_have)
+
+
+
 MHTMLPolicy = email.policy.default.clone(linesep='\r\n', max_line_length=0)
 
 
@@ -250,6 +267,10 @@ class _Downloader():
 
         for element in elements:
             element = webelem.WebElementWrapper(element)
+            # Websites are free to set whatever rel=... attribute they want.
+            # We just care about stylesheets and icons.
+            if not _check_rel(element):
+                continue
             if 'src' in element:
                 element_url = element['src']
             elif 'href' in element: