Vue Devtools
works on all demos/examples online but not on my local pages. Even with the following, the Vue Devtools
icon remains gray ("Vue.js not detected"). Why?
<html>
<head>
<script src="https://cdn.jsdelivr.net/npm/vue"></script>
</head>
<body>
<div id="app"></div>
<script>
Vue.config.devtools = true;
</script>
</body>
</html>
The Vue source you are using there looks to be minimized / production build to me. You need to use the non minimized / non-production build. Try https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.15/vue.js instead.
Also if you are working with local files i.e. accessing a page like file://...
then "you need to check "Allow access to file URLs" for this extension in Chrome's extension management panel." see https://github.com/vuejs/vue-devtools