Incorrect syntax highlighting for keyframes at-rule when using vendor prefixes

[clchen28]

Prerequisites

• Put an X between the brackets on this line if you have done all of the following:
  • Reproduced the problem in Safe Mode: http://flight-manual.atom.io/hacking-atom/sections/debugging/#using-safe-mode
  • Followed all applicable steps in the debugging guide: http://flight-manual.atom.io/hacking-atom/sections/debugging/
  • Checked the FAQs on the message board for common solutions: https://discuss.atom.io/c/faq
  • Checked that your issue isn't already filed: https://github.com/issues?utf8=✓&q=is%3Aissue+user%3Aatom
  • Checked that there is not already an Atom package that provides the described functionality: https://atom.io/packages

Description

The syntax highlighting for the keyframes at-rule is not applied correctly if there are vendor prefixes. e.g. @keyframes produces the correct syntax highlighting, but @-moz-keyframes, @-o-keyframes, @-webkit-keyframes and @-ms-keyframes do not.

Steps to Reproduce

Two ways to reproduce.

First way:

1. Create a new file with a .css extension
2. Paste the following text into this file:

@keyframes fadein {
    from { opacity: 0; }
    to   { opacity: 1; }
}

/* Syntax highlighting incorrect for vendor prefixes
/* Firefox < 16 */
@-moz-keyframes fadein {
    from { opacity: 0; }
    to   { opacity: 1; }
}

/* Safari, Chrome and Opera > 12.1 */
@-webkit-keyframes fadein {
    from { opacity: 0; }
    to   { opacity: 1; }
}

/* Internet Explorer */
@-ms-keyframes fadein {
    from { opacity: 0; }
    to   { opacity: 1; }
}

/* Opera < 12.1 */
@-o-keyframes fadein {
    from { opacity: 0; }
    to   { opacity: 1; }
}

3. Open the file in Atom

Second way:

1. Open a blank file (File > New File)
2. Paste the above text into this blank file
3. Switch the syntax to CSS

Expected behavior:
The following behavior for -webkit-keyframes, -o-keyframes, -moz-keyframes, and -ms-keyframes

Actual behavior:

Reproduces how often: 100%

Versions

Atom : 1.15.0
Electron: 1.3.13
Chrome : 52.0.2743.82
Node : 6.5.0
OS : MacOS Sierra 10.12.3

Additional Information

Tested with Seti and Atom Light syntax themes. Seems reproducible on multiple syntax themes.

[clchen28]

P.S. I haven't contributed to open source before - this is my first time. If this is accepted as a bug, I have a fix to this that I'd love to put a PR in for.