diff --git a/tools/find-add-class b/tools/find-add-class
new file mode 100755
index 0000000000..6903de753e
--- /dev/null
+++ b/tools/find-add-class
@@ -0,0 +1,47 @@
+#!/usr/bin/env python
+from __future__ import absolute_import
+from __future__ import print_function
+
+from lib.find_add_class import display, find
+import glob
+import optparse
+import sys
+
+from six.moves import filter
+try:
+    import lister
+    from typing import cast, Callable, Dict, Iterable, List
+except ImportError as e:
+    print("ImportError: {}".format(e))
+    print("You need to run the Zulip linters inside a Zulip dev environment.")
+    print("If you are using Vagrant, you can `vagrant ssh` to enter the Vagrant guest.")
+    sys.exit(1)
+
+def process_files():
+    # type: () -> None
+
+    usage = '''
+        Use this tool to find HTML classes that we use in our JS code.
+        This looks for calls to addClass, and if you use the -v option,
+        you will get a display of (fn, html_class) tuples that
+        represent addClass calls.
+
+        If you call it with no options, the tool acts as a linter, and
+        it will complain if it can't resolve the class for an addClass()
+        call.
+        '''
+
+    parser = optparse.OptionParser(usage=usage)
+    parser.add_option('--verbose', '-v',
+        action='store_true', default=False,
+        help='Show where calls are.')
+    (options, _) = parser.parse_args()
+
+    fns = glob.glob('static/js/*.js')
+    if options.verbose:
+        display(fns)
+    else:
+        find(fns)
+
+if __name__ == '__main__':
+    process_files()
diff --git a/tools/lib/find_add_class.py b/tools/lib/find_add_class.py
new file mode 100644
index 0000000000..7f2b0c6140
--- /dev/null
+++ b/tools/lib/find_add_class.py
@@ -0,0 +1,98 @@
+from __future__ import absolute_import
+from __future__ import print_function
+
+from typing import Set, Tuple
+
+import os
+import re
+
+GENERIC_KEYWORDS = [
+    'active',
+    'alert',
+    'danger',
+    'condensed',
+    'disabled',
+    'error',
+    'expanded',
+    'hide',
+    'notdisplayed',
+    'popover',
+    'success',
+    'text-error',
+    'warning',
+]
+
+def raise_error(fn, i, line):
+    # type: (str, int, str) -> None
+    error = '''
+        In %s line %d there is the following line of code:
+
+        %s
+
+        Our tools want to be able to identify which modules
+        add which HTML/CSS classes, and we need two things to
+        happen:
+
+            - The code must explicitly name the class.
+            - Only one module can refer to that class (unless
+              it is something generic like an alert class).
+
+        If you get this error, you can usually address it by
+        refactoring your code to be more explicit, or you can
+        move the common code that sets the class to a library
+        module.  If neither of those applies, you need to
+        modify %s
+        ''' % (fn, i, line, __file__)
+    raise Exception(error)
+
+def generic(html_class):
+    # type: (str) -> bool
+    for kw in GENERIC_KEYWORDS:
+        if kw in html_class:
+            return True
+
+def display(fns):
+    # type: (List[str]) -> None
+    for tup in find(fns):
+        # this format is for code generation purposes
+        print(' ' * 8 + repr(tup) + ',')
+
+def find(fns):
+    # type: (List[str]) -> List[Tuple[str, str]]
+    encountered = set() # type: Set[str]
+    tups = [] # type: List[Tuple[str, str]]
+    for fn in fns:
+        lines = list(open(fn))
+        fn = os.path.basename(fn)
+        module_classes = set() # type: Set[str]
+        for i, line in enumerate(lines):
+            if 'addClass' in line:
+                html_classes = [] # type: List[str]
+                m = re.search('addClass\([\'"](.*?)[\'"]', line)
+                if m:
+                    html_classes = [m.group(1)]
+                if not html_classes:
+                    if 'bar-success' in line:
+                        html_classes = ['bar-success', 'bar-danger']
+                    elif 'color_class' in line:
+                        continue
+                    elif 'stream_dark' in line:
+                        continue
+                    elif fn == 'signup.js' and 'class_to_add' in line:
+                        html_classes = ['error', 'success']
+                    elif fn == 'ui.js' and 'status_classes' in line:
+                        html_classes = ['alert']
+
+                if not html_classes:
+                    raise_error(fn, i, line)
+                for html_class in html_classes:
+                    if generic(html_class):
+                        continue
+                    if html_class in module_classes:
+                        continue
+                    if html_class in encountered:
+                        raise_error(fn, i, line)
+                    tups.append((fn, html_class))
+                    module_classes.add(html_class)
+                    encountered.add(html_class)
+    return tups
diff --git a/tools/lint-all b/tools/lint-all
index 1563fb776e..a74f9f15b3 100755
--- a/tools/lint-all
+++ b/tools/lint-all
@@ -401,6 +401,12 @@ try:
         result = subprocess.call(args)
         return result
 
+    @lint
+    def add_class():
+        # type: () -> int
+        result = subprocess.call(['tools/find-add-class'])
+        return result
+
     @lint
     def css():
         # type: () -> int