diff --git a/app/index.html b/app/index.html
new file mode 100644
index 0000000..552818d
--- /dev/null
+++ b/app/index.html
@@ -0,0 +1,69 @@
+<!-- retoor <retoor@molodetz.nl> -->
+<!DOCTYPE html>
+<html lang="en">
+<head>
+<meta charset="UTF-8">
+<meta name="viewport" content="width=device-width, initial-scale=1.0">
+<title>Calculator</title>
+</head>
+<body>
+<h1>Calculator</h1>
+
+<fieldset>
+<legend>Add / Subtract</legend>
+<input type="number" id="left" placeholder="Left operand">
+<input type="number" id="right" placeholder="Right operand">
+<button onclick="calculate('add')">Add</button>
+<button onclick="calculate('subtract')">Subtract</button>
+</fieldset>
+
+<fieldset>
+<legend>Clamp</legend>
+<input type="number" id="value" placeholder="Value">
+<input type="number" id="low" placeholder="Low">
+<input type="number" id="high" placeholder="High">
+<button onclick="calculate('clamp')">Clamp</button>
+</fieldset>
+
+<fieldset>
+<legend>Clamp to Byte</legend>
+<input type="number" id="byte_value" placeholder="Value">
+<button onclick="calculate('clamp_to_byte')">Clamp to Byte</button>
+</fieldset>
+
+<p id="output"></p>
+
+<script>
+function calculate(operation) {
+  const resultEl = document.getElementById('output');
+  let url;
+  if (operation === 'add' || operation === 'subtract') {
+    const left = document.getElementById('left').value;
+    const right = document.getElementById('right').value;
+    url = '/' + operation + '?left=' + encodeURIComponent(left) + '&right=' + encodeURIComponent(right);
+  } else if (operation === 'clamp') {
+    const value = document.getElementById('value').value;
+    const low = document.getElementById('low').value;
+    const high = document.getElementById('high').value;
+    url = '/clamp?value=' + encodeURIComponent(value) + '&low=' + encodeURIComponent(low) + '&high=' + encodeURIComponent(high);
+  } else if (operation === 'clamp_to_byte') {
+    const value = document.getElementById('byte_value').value;
+    url = '/clamp_to_byte?value=' + encodeURIComponent(value);
+  }
+  fetch(url)
+    .then(function(response) {
+      return response.json().then(function(data) {
+        if (!response.ok) {
+          resultEl.textContent = 'Error: ' + (data.error || 'Unknown error');
+        } else {
+          resultEl.textContent = 'Result: ' + data.result;
+        }
+      });
+    })
+    .catch(function() {
+      resultEl.textContent = 'Error: Network error';
+    });
+}
+</script>
+</body>
+</html>
diff --git a/tests/test_api.py b/tests/test_api.py
new file mode 100644
index 0000000..b1502cd
--- /dev/null
+++ b/tests/test_api.py
@@ -0,0 +1,87 @@
+# retoor <retoor@molodetz.nl>
+
+import json
+import unittest
+
+from app import app
+
+
+class TestCalculatorAPI(unittest.TestCase):
+
+    def setUp(self) -> None:
+        self.client = app.test_client()
+
+    def test_add_success(self) -> None:
+        response = self.client.get('/add?left=3&right=5')
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json, {'result': 8})
+
+    def test_add_missing_param_returns_400(self) -> None:
+        response = self.client.get('/add?left=3')
+        self.assertEqual(response.status_code, 400)
+        self.assertIn('error', response.json)
+
+    def test_add_invalid_type_returns_400(self) -> None:
+        response = self.client.get('/add?left=abc&right=5')
+        self.assertEqual(response.status_code, 400)
+        self.assertIn('error', response.json)
+
+    def test_subtract_success(self) -> None:
+        response = self.client.get('/subtract?left=10&right=3')
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json, {'result': 7})
+
+    def test_subtract_missing_param_returns_400(self) -> None:
+        response = self.client.get('/subtract?right=3')
+        self.assertEqual(response.status_code, 400)
+        self.assertIn('error', response.json)
+
+    def test_subtract_invalid_type_returns_400(self) -> None:
+        response = self.client.get('/subtract?left=10&right=xyz')
+        self.assertEqual(response.status_code, 400)
+        self.assertIn('error', response.json)
+
+    def test_clamp_success(self) -> None:
+        response = self.client.get('/clamp?value=5&low=0&high=10')
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json, {'result': 5})
+
+    def test_clamp_value_below_low_returns_low(self) -> None:
+        response = self.client.get('/clamp?value=-5&low=0&high=10')
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json, {'result': 0})
+
+    def test_clamp_missing_param_returns_400(self) -> None:
+        response = self.client.get('/clamp?value=5&low=0')
+        self.assertEqual(response.status_code, 400)
+        self.assertIn('error', response.json)
+
+    def test_clamp_invalid_type_returns_400(self) -> None:
+        response = self.client.get('/clamp?value=abc&low=0&high=10')
+        self.assertEqual(response.status_code, 400)
+        self.assertIn('error', response.json)
+
+    def test_clamp_low_greater_than_high_returns_400(self) -> None:
+        response = self.client.get('/clamp?value=5&low=10&high=0')
+        self.assertEqual(response.status_code, 400)
+        self.assertIn('error', response.json)
+
+    def test_clamp_to_byte_success(self) -> None:
+        response = self.client.get('/clamp_to_byte?value=100')
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json, {'result': 100})
+
+    def test_clamp_to_byte_above_255_returns_255(self) -> None:
+        response = self.client.get('/clamp_to_byte?value=300')
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json, {'result': 255})
+
+    def test_clamp_to_byte_missing_param_returns_400(self) -> None:
+        response = self.client.get('/clamp_to_byte')
+        self.assertEqual(response.status_code, 400)
+        self.assertIn('error', response.json)
+
+    def test_clamp_to_byte_invalid_type_returns_400(self) -> None:
+        response = self.client.get('/clamp_to_byte?value=abc')
+        self.assertEqual(response.status_code, 400)
+        self.assertIn('error', response.json)
