[web_server] Show a hint when the hosted web interface cannot be downloaded

tests/unit_tests/components/test_web_server.py

@@ -0,0 +1,28 @@
+"""Tests for the web_server component index page builder."""
+
+from esphome.components.web_server import build_index_html
+from esphome.const import CONF_CSS_URL, CONF_JS_URL
+
+
+def test_build_index_html_has_offline_hint() -> None:
+    """The hosted script tag shows a hint when the browser cannot download it."""
+    html = build_index_html(
+        {CONF_JS_URL: "https://oi.esphome.io/v2/www.js", CONF_CSS_URL: ""}
+    )
+    assert '<script src="https://oi.esphome.io/v2/www.js" onerror="' in html
+    assert "local: true" in html
+    assert "https://oi.esphome.io/v2/www.js. This browser needs internet" in html
+
+
+def test_build_index_html_escapes_hint() -> None:
+    """Quotes in the script URL cannot break out of the onerror attribute."""
+    html = build_index_html({CONF_JS_URL: "http://x/a'b\"c.js", CONF_CSS_URL: ""})
+    assert "onerror=\"document.body.innerText='" in html
+    assert "a\\&#x27;b&quot;c.js" in html
+
+
+def test_build_index_html_without_js_url() -> None:
+    """No hosted script and no hint when js_url is empty."""
+    html = build_index_html({CONF_JS_URL: "", CONF_CSS_URL: ""})
+    assert "onerror" not in html
+    assert "<script" not in html