Add mixlr.com to the oEmbed whitelist

[mrgeorgegray]

Added mixlr.com to wp-includes/class-oembed.php

[nacin]

We have a certain standard for oEmbed providers in core. They must be well-established, popular, mainstream services; they must properly implement the oEmbed spec; and they must clearly be a trusted provider. Trust is more than about security — the service must also be "built to last," as if a provider ever stops supporting oEmbed, sites start to lose content they previously trusted would stay embedded permanently. Some nascent services are just too fragile to be material for core.

With regards to establishment and popularity, there are a number of "warning signs":

• I've personally never heard of Mixlr
• While it is for "social live audio", it didn't come up during WordPress 3.5 or 3.6, which added a lot of support for audio/video, including Spotify, Rdio, and Soundcloud via oEmbed
• I can't find an article about it on Wikipedia
• Even though it is a "social" platform, I almost have as many followers as @Mixlr on Twitter, and it pales in comparison to other aforementioned giants
• It is less than a year old (at least its reincarnation)
• The oEmbed endpoint appears undocumented: ​https://www.google.com/search?q=site:mixlr.com+oembed

I see you have just today ​added a shortcode plugin to WordPress.org. It is best to see what kind of traction/adoption it or any other plugins may receive. (Perhaps notably, it's the only result when searching for "mixlr" in the directory.)

[mrgeorgegray]

Thanks Nacin. We fully take on board everything you've said here. Let's see how the new plugin goes and we'll work on the other points you mentioned and perhaps review this again further down the line.

[nacin]

I turned my comment above into a section on our nascent design decisions page: ​http://make.wordpress.org/core/handbook/design-decisions/#whitelisting-oembed-providers.