diff --git a/lib/rules/no-unknown-property.js b/lib/rules/no-unknown-property.js
index 18634951c3..6b4c87ec4c 100644
--- a/lib/rules/no-unknown-property.js
+++ b/lib/rules/no-unknown-property.js
@@ -147,7 +147,8 @@ function getDOMPropertyNames(context) {
 // ------------------------------------------------------------------------------
 
 /**
- * Checks if a node matches the JSX tag convention.
+ * Checks if a node matches the JSX tag convention. This also checks if a node
+ * is extended as a webcomponent using the attribute "is".
  * @param {Object} node - JSX element being tested.
  * @returns {boolean} Whether or not the node name match the JSX tag convention.
  */
@@ -194,7 +195,7 @@ function tagNameHasDot(node) {
  * Get the standard name of the attribute.
  * @param {String} name - Name of the attribute.
  * @param {String} context - eslint context
- * @returns {String} The standard name of the attribute.
+ * @returns {String | null} The standard name of the attribute, or null if no standard name was found.
  */
 function getStandardName(name, context) {
   if (DOM_ATTRIBUTE_NAMES[name]) {
@@ -205,11 +206,11 @@ function getStandardName(name, context) {
   }
   let i = -1;
   const names = getDOMPropertyNames(context);
-  const found = names.some((element, index) => {
-    i = index;
-    return element.toLowerCase() === name;
-  });
-  return found ? names[i] : null;
+  // Let's find a possible attribute match with a case-insensitive search.
+  const foundName = names.find(
+    (element, index) => element.toLowerCase() === name.toLowerCase()
+  );
+  return foundName === undefined ? null : foundName;
 }
 
 // ------------------------------------------------------------------------------
@@ -259,6 +260,8 @@ module.exports = {
         }
 
         const tagName = getTagName(node);
+
+        // 1. Some attributes are allowed on some tags only.
         const allowedTags = ATTRIBUTE_TAGS_MAP[name];
         if (tagName && allowedTags && /[^A-Z]/.test(tagName.charAt(0)) && allowedTags.indexOf(tagName) === -1) {
           context.report({
@@ -272,8 +275,12 @@ module.exports = {
           });
         }
 
+        // 2. Otherwise, we'll try to find if the attribute is a close version
+        // of what we should normally have with React. If yes, we'll report an
+        // error. We don't want to report if the input attribute name is the
+        // standard name though!
         const standardName = getStandardName(name, context);
-        if (!isTagName(node) || !standardName) {
+        if (!isTagName(node) || !standardName || standardName === name) {
           return;
         }
         context.report({
diff --git a/tests/lib/rules/no-unknown-property.js b/tests/lib/rules/no-unknown-property.js
index 21197a4e06..a635982873 100644
--- a/tests/lib/rules/no-unknown-property.js
+++ b/tests/lib/rules/no-unknown-property.js
@@ -75,6 +75,10 @@ ruleTester.run('no-unknown-property', rule, {
     code: '<div onmousedown="bar"></div>;',
     output: '<div onMouseDown="bar"></div>;',
     errors: [{message: 'Unknown property \'onmousedown\' found, use \'onMouseDown\' instead'}]
+  }, {
+    code: '<div onMousedown="bar"></div>;',
+    output: '<div onMouseDown="bar"></div>;',
+    errors: [{message: 'Unknown property \'onMousedown\' found, use \'onMouseDown\' instead'}]
   }, {
     code: '<use xlink:href="bar" />;',
     output: '<use xlinkHref="bar" />;',