wp_style_add_data() doesn't support "downlevel-revealed" conditional comments

[pburke]

When used to add conditional comments to enqueued stylesheets, wp_style_add_data() generates code that works great for targeting specific versions of Internet Explorer ('lt IE 9' etc.), but does not work for targeting non-IE browsers ('!IE' or '!IEMobile').

Those types of conditional comments are called "downlevel-revealed" comments, and need a specific syntax so they are read by non-IE browsers but hidden by IE:

<!--[if !IE]><!-->
  ...
<!--<![endif]-->

The syntax generated by wp_style_add_data() is:

<!--[if !IE]>
...
<![endif]-->

... which causes the contained code to be ignored by non-IE browsers.

To reproduce:
1) In a theme's functions.php file, use wp_style_add_data() to attach a conditional comment that targets non-IE browsers to the theme's stylesheet:

wp_enqueue_style( 'mytheme-style', get_stylesheet_uri(), array(), '2013-09-27' );
wp_style_add_data( 'mytheme-style', 'conditional', '!IE' );

2) Enable the theme and view the page in a non-IE browser. Notice that the browser ignores the stylesheet declaration.

Reference: ​downlevel-revealed versus downlevel-hidden conditional comments.

[SergeyBiryukov]

#16118, #21162, #22122