diff --git a/common/lib/capa/capa/templates/textline.html b/common/lib/capa/capa/templates/textline.html
index 58bba7d093..f927d732f7 100644
--- a/common/lib/capa/capa/templates/textline.html
+++ b/common/lib/capa/capa/templates/textline.html
@@ -23,10 +23,7 @@
 % for description_id, description_text in response_data['descriptions'].items():
     <p class="question-description" id="${description_id}">${description_text}</p>
 % endfor
-<input type="text" name="input_${id}" id="input_${id}" ${describedby_html} value="${value}"
-    % if do_math:
-        class="math"
-    % endif
+<input type="text" class="mw-100 ${'math' if do_math else ''}" name="input_${id}" id="input_${id}" ${describedby_html} value="${value}"
     % if size:
         size="${size}"
     % endif
diff --git a/common/lib/capa/capa/tests/test_input_templates.py b/common/lib/capa/capa/tests/test_input_templates.py
index 3425c623d0..d18f7fc491 100644
--- a/common/lib/capa/capa/tests/test_input_templates.py
+++ b/common/lib/capa/capa/tests/test_input_templates.py
@@ -542,7 +542,7 @@ class TextlineTemplateTest(TemplateTestCase):
         self.context['do_math'] = True
         xml = self.render_to_xml(self.context)
 
-        xpath = "//input[@class='math']"
+        xpath = "//input[@class='mw-100 math']"
         self.assert_has_xpath(xml, xpath, self.context)
 
         xpath = "//div[@class='equation']"