Mailchimp's custom attributes allows anyone to add, change or remove without touching code through their design view.
When you're using React or a module that uses it as dependency (ie: react-html-email), it will error since mc:edit isn't valid. This replaces any string that includes data-mc-edit with Mailchimp's mc:edit.
data-mc-edit
data-mc-hideable
data-mc-repeatable
data-mc-variant
data-mc-labelnpm install --save mailchimpifyvar mailchimpify = require('mailchimpify');
var div = '<div data-mc-edit="content">some text</div>';
mailchimpify(div); // <div mc:edit="content">some text</div>;MIT%