The web platform has evolved organically over the past two decades, slowly growing new capabilities and APIs. Many features that are added are great ideas that enable web developers to make even better applications. But some APIs turn out, in retrospect, to be mistakes. Over time, the platform accretes more bad APIs, which makes it harder to add new browser features, confuses web developers, and even introduces security bugs. showModalDialog is a bad API that we deprecated earlier this year, and in Chrome 37 we will disable support for it by default.

showModalDialog was first introduced in Internet Explorer 4 and although it was never formally standardized, over time most other browsers added support for it. It allows applications to show a dialog of HTML content that freezes all other content while showing. showModalDialog is not a commonly used API: based on our usage counters, less than 0.006% of pages use it.

Unfortunately, showModalDialog's unique ability to freeze content is now widely regarded as a mis-feature in terms of user experience, code complexity, and security. From a usability perspective, showModalDialog rudely demands that you interact with it by freezing all of your other tabs—even ones from other sites. showModalDialog also requires complex and hard-to-maintain code scattered throughout the codebase. This complexity complicates the behavior of new web features like Mutation Observers, Object.observe, and Promises. It also makes showModalDialog a source of a disproportionate number of bugs, including serious security vulnerabilities. It is for these reasons that we decided to turn off showModalDialog by default in the next version of Chrome.

Although very few sites use showModalDialog, the small minority that do—disproportionately enterprise sites—have come to rely heavily on it. In order to give these sites more time to update, we have added a temporary Enterprise Policy setting to re-enable showModalDialog. In May 2015 this setting will be removed and showModalDialog will be completely removed from Chromium. Affected sites should begin work to update their sites as soon as possible.

Although it can be difficult, sometimes the only way to go forward is to leave the past behind. Removing bad APIs will help us make the web a more consistent and capable platform for both developers and users.

Posted by Adam Barth, Software Engineer