diff --git a/src/calibre/ebooks/oeb/polish/check/css.py b/src/calibre/ebooks/oeb/polish/check/css.py
index 9e4087013f..9b6812e45a 100644
--- a/src/calibre/ebooks/oeb/polish/check/css.py
+++ b/src/calibre/ebooks/oeb/polish/check/css.py
@@ -63,7 +63,7 @@ def message_to_error(message, name, line_offset=0):
     ans.HELP = rule.get('desc') or ''
     ans.css_rule_id = rule_id
     if ans.HELP and 'url' in rule:
-        ans.HELP += ' ' + _('See {}').format(rule['url'])
+        ans.HELP += ' ' + _('See <a href="{}">detailed description</a>.').format(rule['url'])
     return ans
 
 
@@ -237,8 +237,10 @@ def create_job(name, css, line_offset=0, is_declaration=False):
 
 
 def check_css(jobs):
-    results = pool.check_css([j.css for j in jobs])
     errors = []
+    if not jobs:
+        return errors
+    results = pool.check_css([j.css for j in jobs])
     for job, result in zip(jobs, results):
         if isinstance(result, dict):
             for msg in result['messages']:
diff --git a/src/calibre/gui2/tweak_book/check.py b/src/calibre/gui2/tweak_book/check.py
index 04f4bdfc27..86dd396fc7 100644
--- a/src/calibre/gui2/tweak_book/check.py
+++ b/src/calibre/gui2/tweak_book/check.py
@@ -13,7 +13,7 @@ from PyQt5.Qt import (
 
 from calibre.ebooks.oeb.polish.check.base import WARN, INFO, DEBUG, ERROR, CRITICAL
 from calibre.ebooks.oeb.polish.check.main import run_checks, fix_errors
-from calibre.gui2 import NO_URL_FORMATTING
+from calibre.gui2 import NO_URL_FORMATTING, safe_open_url
 from calibre.gui2.tweak_book import tprefs
 from calibre.gui2.tweak_book.widgets import BusyCursor
 from polyglot.builtins import unicode_type, range
@@ -131,6 +131,8 @@ class Check(QSplitter):
         elif url.startswith('activate:item:'):
             index = int(url.rpartition(':')[-1])
             self.location_activated(index)
+        elif url.startswith('https://'):
+            safe_open_url(url)
 
     def next_error(self, delta=1):
         row = self.items.currentRow()