diff --git a/src/validate/html/index.js b/src/validate/html/index.js
index f54c4d4fe5..8cad9bd849 100644
--- a/src/validate/html/index.js
+++ b/src/validate/html/index.js
@@ -1,7 +1,13 @@
 import * as namespaces from '../../utils/namespaces.js';
+import flattenReference from '../../utils/flattenReference.js';
 
 const svg = /^(?:altGlyph|altGlyphDef|altGlyphItem|animate|animateColor|animateMotion|animateTransform|circle|clipPath|color-profile|cursor|defs|desc|discard|ellipse|feBlend|feColorMatrix|feComponentTransfer|feComposite|feConvolveMatrix|feDiffuseLighting|feDisplacementMap|feDistantLight|feDropShadow|feFlood|feFuncA|feFuncB|feFuncG|feFuncR|feGaussianBlur|feImage|feMerge|feMergeNode|feMorphology|feOffset|fePointLight|feSpecularLighting|feSpotLight|feTile|feTurbulence|filter|font|font-face|font-face-format|font-face-name|font-face-src|font-face-uri|foreignObject|g|glyph|glyphRef|hatch|hatchpath|hkern|image|line|linearGradient|marker|mask|mesh|meshgradient|meshpatch|meshrow|metadata|missing-glyph|mpath|path|pattern|polygon|polyline|radialGradient|rect|set|solidcolor|stop|switch|symbol|text|textPath|title|tref|tspan|unknown|use|view|vkern)$/;
 
+function list ( items, conjunction = 'or' ) {
+	if ( items.length === 1 ) return items[0];
+	return `${items.slice( 0, -1 ).join( ', ' )} ${conjunction} ${items[ items.length - 1 ]}`;
+}
+
 export default function validateHtml ( validator, html ) {
 	let elementDepth = 0;
 
@@ -12,6 +18,30 @@ export default function validateHtml ( validator, html ) {
 			}
 
 			elementDepth += 1;
+
+			node.attributes.forEach( attribute => {
+				if ( attribute.type === 'EventHandler' ) {
+					const { callee, start, type } = attribute.expression;
+
+					if ( type !== 'CallExpression' ) {
+						validator.error( `Expected a call expression`, start );
+					}
+
+					const { name } = flattenReference( callee );
+
+					if ( name === 'this' || name === 'event' ) return;
+					if ( callee.type === 'Identifier' && callee.name === 'set' || callee.name === 'fire' || callee.name in validator.methods ) return;
+
+					const validCallees = list( [ 'this.*', 'event.*', 'set', 'fire' ].concat( Object.keys( validator.methods ) ) );
+					let message = `'${validator.source.slice( callee.start, callee.end )}' is an invalid callee (should be one of ${validCallees})`;
+
+					if ( callee.type === 'Identifier' && callee.name in validator.helpers ) {
+						message += `. '${callee.name}' exists on 'helpers', did you put it in the wrong place?`;
+					}
+
+					validator.error( message, start );
+				}
+			});
 		}
 
 		if ( node.children ) {
diff --git a/src/validate/index.js b/src/validate/index.js
index 4efdee8d47..f0a274c9eb 100644
--- a/src/validate/index.js
+++ b/src/validate/index.js
@@ -36,9 +36,13 @@ export default function validate ( parsed, source, { onerror, onwarn, name, file
 			});
 		},
 
+		source,
+
 		namespace: null,
 		defaultExport: null,
-		properties: {}
+		properties: {},
+		methods: {},
+		helpers: {}
 	};
 
 	if ( name && !/^[a-zA-Z_$][a-zA-Z_$0-9]*$/.test( name ) ) {
diff --git a/src/validate/js/index.js b/src/validate/js/index.js
index d0fc396320..3f7e8c0e42 100644
--- a/src/validate/js/index.js
+++ b/src/validate/js/index.js
@@ -72,4 +72,12 @@ export default function validateJs ( validator, js ) {
 			});
 		}
 	});
+
+	[ 'methods', 'helpers' ].forEach( key => {
+		if ( validator.properties[ key ] ) {
+			validator.properties[ key ].value.properties.forEach( prop => {
+				validator[ key ][ prop.key.name ] = prop.value;
+			});
+		}
+	});
 }
diff --git a/test/validator/samples/method-nonexistent-helper/errors.json b/test/validator/samples/method-nonexistent-helper/errors.json
new file mode 100644
index 0000000000..80183498ca
--- /dev/null
+++ b/test/validator/samples/method-nonexistent-helper/errors.json
@@ -0,0 +1,8 @@
+[{
+	"message": "'foo' is an invalid callee (should be one of this.*, event.*, set, fire or bar). 'foo' exists on 'helpers', did you put it in the wrong place?",
+	"pos": 18,
+	"loc": {
+		"line": 1,
+		"column": 18
+	}
+}]
diff --git a/test/validator/samples/method-nonexistent-helper/input.html b/test/validator/samples/method-nonexistent-helper/input.html
new file mode 100644
index 0000000000..abb756f6c3
--- /dev/null
+++ b/test/validator/samples/method-nonexistent-helper/input.html
@@ -0,0 +1,17 @@
+<button on:click='foo()'></button>
+
+<script>
+	export default {
+		methods: {
+			bar () {
+				// ...
+			}
+		},
+
+		helpers: {
+			foo ( x ) {
+				return x;
+			}
+		}
+	};
+</script>
diff --git a/test/validator/samples/method-nonexistent/errors.json b/test/validator/samples/method-nonexistent/errors.json
new file mode 100644
index 0000000000..2eaf6c215d
--- /dev/null
+++ b/test/validator/samples/method-nonexistent/errors.json
@@ -0,0 +1,8 @@
+[{
+	"message": "'foo' is an invalid callee (should be one of this.*, event.*, set, fire or bar)",
+	"pos": 18,
+	"loc": {
+		"line": 1,
+		"column": 18
+	}
+}]
diff --git a/test/validator/samples/method-nonexistent/input.html b/test/validator/samples/method-nonexistent/input.html
new file mode 100644
index 0000000000..75f2028b92
--- /dev/null
+++ b/test/validator/samples/method-nonexistent/input.html
@@ -0,0 +1,11 @@
+<button on:click='foo()'></button>
+
+<script>
+	export default {
+		methods: {
+			bar () {
+				// ...
+			}
+		}
+	};
+</script>