diff --git a/benefits/core/templates/core/includes/form.html b/benefits/core/templates/core/includes/form.html
index ed4ec3b08..cdf69abac 100644
--- a/benefits/core/templates/core/includes/form.html
+++ b/benefits/core/templates/core/includes/form.html
@@ -59,7 +59,29 @@
         $("#{{ form.id }}").on("submit", function(e) {
           $(this).trigger("submitting");
         });
-      });
+
+        {% if form.use_custom_validity %}
+        const validate = function(input_element) {
+          input_element.setCustomValidity(""); // clearing message sets input_element.validity.customError back to false
+
+          const valid = input_element.checkValidity();
+          if (!valid) {
+            input_element.setCustomValidity(input_element.dataset.customValidity);
+          }
+        }
+
+        $("button[type=submit]", "#{{ form.id }}").on("click", function(e) {
+            // revalidate all fields
+            $("[data-custom-validity]").each(function() {
+              let input_element = $(this)[0];
+              if (input_element) {
+                validate(input_element);
+              }
+            });
+        });
+
+        {% endif %}
+    });
     </script>
 
     {% if request.recaptcha %}
diff --git a/benefits/eligibility/forms.py b/benefits/eligibility/forms.py
index a7ff58dbe..d66ee506b 100644
--- a/benefits/eligibility/forms.py
+++ b/benefits/eligibility/forms.py
@@ -30,8 +30,11 @@ def __init__(self, agency: models.TransitAgency, *args, **kwargs):
         self.classes = "col-lg-8"
         # second element is not used since we render the whole label using selection_label_template,
         # therefore set to None
-        self.fields["verifier"].choices = [(v.id, None) for v in verifiers]
-        self.fields["verifier"].widget.selection_label_templates = {v.id: v.selection_label_template for v in verifiers}
+        verifier_field = self.fields["verifier"]
+        verifier_field.choices = [(v.id, None) for v in verifiers]
+        verifier_field.widget.selection_label_templates = {v.id: v.selection_label_template for v in verifiers}
+        verifier_field.widget.attrs.update({"data-custom-validity": _("Please choose a transit benefit.")})
+        self.use_custom_validity = True
 
     def clean(self):
         if not recaptcha.verify(self.data):
@@ -48,11 +51,6 @@ class EligibilityVerificationForm(forms.Form):
     submit_value = _("Find my record")
     submitting_value = _("Checking")
 
-    _error_messages = {
-        "invalid": _("Check your input. The format looks wrong."),
-        "missing": _("This field is required."),
-    }
-
     def __init__(
         self,
         title,
@@ -68,6 +66,8 @@ def __init__(
         sub_input_mode=None,
         sub_max_length=None,
         sub_pattern=None,
+        sub_custom_validity=None,
+        name_custom_validity=None,
         *args,
         **kwargs,
     ):
@@ -117,6 +117,9 @@ def __init__(
             sub_widget.attrs.update({"inputmode": sub_input_mode})
         if sub_max_length:
             sub_widget.attrs.update({"maxlength": sub_max_length})
+        if sub_custom_validity:
+            sub_widget.attrs.update({"data-custom-validity": sub_custom_validity})
+            self.use_custom_validity = True
 
         self.fields["sub"] = forms.CharField(
             label=sub_label,
@@ -127,6 +130,9 @@ def __init__(
         name_widget = widgets.FormControlTextInput(placeholder=name_placeholder)
         if name_max_length:
             name_widget.attrs.update({"maxlength": name_max_length})
+        if name_custom_validity:
+            name_widget.attrs.update({"data-custom-validity": name_custom_validity})
+            self.use_custom_validity = True
 
         self.fields["name"] = forms.CharField(label=name_label, widget=name_widget, help_text=name_help_text)
 
@@ -157,6 +163,8 @@ def __init__(self, *args, **kwargs):
             sub_input_mode="numeric",
             sub_max_length=5,
             sub_pattern=r"\d{5}",
+            sub_custom_validity=_("Please enter a 5-digit number."),
+            name_custom_validity=_("Please enter your last name."),
             *args,
             **kwargs,
         )
@@ -185,6 +193,8 @@ def __init__(self, *args, **kwargs):
             sub_input_mode="numeric",
             sub_max_length=4,
             sub_pattern=r"\d{4}",
+            sub_custom_validity=_("Please enter a 4-digit number."),
+            name_custom_validity=_("Please enter your last name."),
             *args,
             **kwargs,
         )
diff --git a/benefits/locale/en/LC_MESSAGES/django.po b/benefits/locale/en/LC_MESSAGES/django.po
index d2b96190b..ac7831352 100644
--- a/benefits/locale/en/LC_MESSAGES/django.po
+++ b/benefits/locale/en/LC_MESSAGES/django.po
@@ -6,7 +6,7 @@
 msgid ""
 msgstr ""
 "Report-Msgid-Bugs-To: https://github.com/cal-itp/benefits/issues \n"
-"POT-Creation-Date: 2024-04-22 12:32-0700\n"
+"POT-Creation-Date: 2024-04-25 12:29-0700\n"
 "Language: English\n"
 "MIME-Version: 1.0\n"
 "Content-Type: text/plain; charset=UTF-8\n"
@@ -303,16 +303,13 @@ msgstr ""
 msgid "Choose this benefit"
 msgstr ""
 
-msgid "Find my record"
-msgstr ""
-
-msgid "Checking"
+msgid "Please choose a transit benefit."
 msgstr ""
 
-msgid "Check your input. The format looks wrong."
+msgid "Find my record"
 msgstr ""
 
-msgid "This field is required."
+msgid "Checking"
 msgstr ""
 
 msgid "Agency card information"
@@ -340,6 +337,12 @@ msgstr ""
 msgid "This is a 5-digit number on the front and back of your card."
 msgstr ""
 
+msgid "Please enter a 5-digit number."
+msgstr ""
+
+msgid "Please enter your last name."
+msgstr ""
+
 msgid ""
 "We use the information on your SBMTD Reduced Fare Mobility ID card to find "
 "the record of your transit benefit in our system."
@@ -351,6 +354,9 @@ msgstr ""
 msgid "This is a 4-digit number on the back of your card."
 msgstr ""
 
+msgid "Please enter a 4-digit number."
+msgstr ""
+
 msgid "Your contactless card details"
 msgstr ""
 
diff --git a/benefits/locale/es/LC_MESSAGES/django.po b/benefits/locale/es/LC_MESSAGES/django.po
index 3da76c2fd..dde67440d 100644
--- a/benefits/locale/es/LC_MESSAGES/django.po
+++ b/benefits/locale/es/LC_MESSAGES/django.po
@@ -6,7 +6,7 @@
 msgid ""
 msgstr ""
 "Report-Msgid-Bugs-To: https://github.com/cal-itp/benefits/issues \n"
-"POT-Creation-Date: 2024-04-22 12:32-0700\n"
+"POT-Creation-Date: 2024-04-25 12:29-0700\n"
 "Language: Español\n"
 "MIME-Version: 1.0\n"
 "Content-Type: text/plain; charset=UTF-8\n"
@@ -390,18 +390,15 @@ msgstr "¿A qué beneficio de tránsito le gustaría inscribirse?"
 msgid "Choose this benefit"
 msgstr "Elegir este beneficio"
 
+msgid "Please choose a transit benefit."
+msgstr ""
+
 msgid "Find my record"
 msgstr "Comprobar elegibilidad"
 
 msgid "Checking"
 msgstr "Comprobando"
 
-msgid "Check your input. The format looks wrong."
-msgstr "Verifique su entrada. El formato parece incorrecto."
-
-msgid "This field is required."
-msgstr "Este campo es requerido."
-
 msgid "Agency card information"
 msgstr "Información de la tarjeta de agencia"
 
@@ -427,6 +424,12 @@ msgstr "Número de tarjeta de cortesía de MST"
 msgid "This is a 5-digit number on the front and back of your card."
 msgstr "Este es un número de 5 dígitos en el anverso y reverso de su tarjeta."
 
+msgid "Please enter a 5-digit number."
+msgstr ""
+
+msgid "Please enter your last name."
+msgstr ""
+
 msgid ""
 "We use the information on your SBMTD Reduced Fare Mobility ID card to find "
 "the record of your transit benefit in our system."
@@ -438,6 +441,9 @@ msgstr "Número de SBMTD Reduced Fare Mobility ID"
 msgid "This is a 4-digit number on the back of your card."
 msgstr "Este es un número de 4 dígitos en el reverso de su tarjeta."
 
+msgid "Please enter a 4-digit number."
+msgstr ""
+
 msgid "Your contactless card details"
 msgstr "Los datos de su tarjeta sin contacto"
 
diff --git a/tests/pytest/eligibility/test_forms.py b/tests/pytest/eligibility/test_forms.py
new file mode 100644
index 000000000..42799ebac
--- /dev/null
+++ b/tests/pytest/eligibility/test_forms.py
@@ -0,0 +1,37 @@
+from benefits.eligibility.forms import MSTCourtesyCard, SBMTDMobilityPass
+
+
+def test_MSTCourtesyCard():
+    form = MSTCourtesyCard(data={"sub": "12345", "name": "Gonzalez"})
+
+    assert form.is_valid()
+
+    sub_attrs = form.fields["sub"].widget.attrs
+    assert sub_attrs["pattern"] == r"\d{5}"
+    assert sub_attrs["inputmode"] == "numeric"
+    assert sub_attrs["maxlength"] == 5
+    assert sub_attrs["data-custom-validity"] == "Please enter a 5-digit number."
+
+    name_attrs = form.fields["name"].widget.attrs
+    assert name_attrs["maxlength"] == 255
+    assert name_attrs["data-custom-validity"] == "Please enter your last name."
+
+    assert form.use_custom_validity
+
+
+def test_SBMTDMobilityPass():
+    form = SBMTDMobilityPass(data={"sub": "1234", "name": "Barbara"})
+
+    assert form.is_valid()
+
+    sub_attrs = form.fields["sub"].widget.attrs
+    assert sub_attrs["pattern"] == r"\d{4}"
+    assert sub_attrs["maxlength"] == 4
+    assert sub_attrs["inputmode"] == "numeric"
+    assert sub_attrs["data-custom-validity"] == "Please enter a 4-digit number."
+
+    name_attrs = form.fields["name"].widget.attrs
+    assert name_attrs["maxlength"] == 255
+    assert name_attrs["data-custom-validity"] == "Please enter your last name."
+
+    assert form.use_custom_validity