Warn if svg uses the viewbox attribute

src/validate/html/validateElement.ts

@@ -79,6 +79,17 @@ export default function validateElement(
 		});
 	}
 
+	if (node.name === 'svg') {
+		node.attributes.forEach(attribute => {
+			if (attribute.name === 'viewbox') {
+				validator.warn(
+					`<svg> has an invalid viewbox attribute – did you mean viewBox?`,
+					node.start
+				)
+			}
+		})
+	}
+
 	let hasIntro: boolean;
 	let hasOutro: boolean;
 	let hasTransition: boolean;

test/validator/samples/svg-viewbox/input.html

@@ -0,0 +1,6 @@
+<svg viewbox="-250 -250 500 500">
+  <line x1="0" y1="0" x2="200" y2="0" stroke="black" />
+  <line x1="0" y1="0" x2="-200" y2="0" stroke="black" />
+  <line x1="0" y1="0" x2="0" y2="200" stroke="black" />
+  <line x1="0" y1="0" x2="0" y2="-200" stroke="black" />
+</svg>

test/validator/samples/svg-viewbox/warnings.json

@@ -0,0 +1,8 @@
+[{
+  "message": "<svg> has an invalid viewbox attribute – did you mean viewBox?",
+  "loc": {
+    "line": 1,
+    "column": 0
+  },
+  "pos": 0
+}]