This is perhaps a minor point, and I only noticed it because I wanted to quickly hide a few fields that site editors weren't ever going to use. But the Twitter cards–related fields have CSS classes on their divs that are invalid. For instance:

form-item-metatags-twitter:creator:id-value
form-item-metatags-twitter:image-value
form-item-metatags-twitter:image:width-value
form-item-metatags-twitter:player:stream:content-type-value

None of those colons are valid in the CSS class names. I'm guessing the field names are just being converted to class names without replacing those colons with hyphens (or something).

Files: 
CommentFileSizeAuthor
#2 drupal-formbuilder_classname_symbols-2058569-3.patch725 bytesrootwork
FAILED: [[SimpleTest]]: [MySQL] Unable to apply patch drupal-formbuilder_classname_symbols-2058569-3.patch. Unable to apply patch. See the log in the details link for more information.
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Title:Twitter card field CSS class names are invalidTwitter card, DC field CSS class names are invalid

Actually I just noticed the Dublin Core fields have invalid class names too:

form-item-metatags-dcterms.subject-value
form-item-metatags-dcterms.contributor-value
form-item-metatags-dcterms.format-value

(etc.)

The periods aren't valid in CSS class names (as they have a functional purpose in stylesheets).

Title:Twitter card, DC field CSS class names are invalidVertical tab class names contain invalid characters
Issue summary:View changes
Status:Active» Needs work
StatusFileSize
new725 bytes
FAILED: [[SimpleTest]]: [MySQL] Unable to apply patch drupal-formbuilder_classname_symbols-2058569-3.patch. Unable to apply patch. See the log in the details link for more information.
[ View ]

I did some more digging. It looks like it's Drupal's formbuilder itself, which only replaces the characters " ", "_", "[", and "]".

I looked through metatag.module, metatag.vertical-tabs.js, and metatag_twitter_cards.metatag.inc and couldn't figure out where it's being called precisely (this isn't my strong suit).

For reference, I'm attaching what would be a patch of the core formbuilder. If Metatag's maintainers think this should actually be handled in core itself, I'll change the project, but really this is just to move this issue along and provide the context so it can be addressed in Metatag.

Status:Needs work» Needs review

Status:Needs review» Active

Correcting the status - the patch isn't for Metatag.