I was having trouble adding custom icons to the TinyMCE toolbar (that's another story for another day), so I thought I'd try adding items to TinyMCE's "Formats" drop down. However, I seem to be running into some issues:

As you can see, two of the three "Uncommon" formats are disabled (namely, abbreviation and quote). The HTML elements these correspond to are "abbr" and "q". The HTML element for "Sample Output" is "samp". Here's how I configured these in my tinyMceConfig.config:

<config key="style_formats">[
  {
    "title": "Headers",
    "items": [
      {"title": "Largest", "format": "h2"},
      {"title": "Large", "format": "h3"},
      {"title": "Medium", "format": "h4"},
      {"title": "Small", "format": "h5"},
      {"title": "Smallest", "format": "h6"}
    ]
  },
  {
    "title": "Uncommon",
    "items": [
      {"title": "Abbreviation", "format": "abbr"},
      {"title": "Quote", "format": "q"},
      {"title": "Sample Output", "format": "samp"}
    ]
  }
]</config>

In that same file, I have also indicated that all three are valid elements (near the bottom of the list):

  <validElements>
    <![CDA TA[+a[id|style|rel|data-id|data-udi|rev|charset|hreflang|dir|lang|tabindex|accesskey|type|name|href|target|title|class|onfocus|onblur|onclick|
ondblclick|onmousedown|onmouseup|onmouseover|onmousemove|onmouseout|onkeypress|onkeydown|onkeyup],-strong/-b[class|style],-em/-i[class|style],
-strike[class|style],-u[class|style],#p[id|style|dir|class|align],-ol[class|reversed|start|style|type],-ul[class|style],-li[class|style],br[class],
img[id|dir|lang|longdesc|usemap|style|class|src|onmouseover|onmouseout|border|alt=|title|hspace|vspace|width|height|align|umbracoorgwidth|umbracoorgheight|onresize|onresizestart|onresizeend|rel|data-id],
-sub[style|class],-sup[style|class],-blockquote[dir|style|class],-table[border=0|cellspacing|cellpadding|width|height|class|align|summary|style|dir|id|lang|bgcolor|background|bordercolor],
-tr[id|lang|dir|class|rowspan|width|height|align|valign|style|bgcolor|background|bordercolor],tbody[id|class],
thead[id|class],tfoot[id|class],#td[id|lang|dir|class|colspan|rowspan|width|height|align|valign|style|bgcolor|background|bordercolor|scope],
-th[id|lang|dir|class|colspan|rowspan|width|height|align|valign|style|scope],caption[id|lang|dir|class|style],-div[id|dir|class|align|style],
-span[class|align|style],-pre[class|align|style],address[class|align|style],-h1[id|dir|class|align|style],-h2[id|dir|class|align|style],
-h3[id|dir|class|align|style],-h4[id|dir|class|align|style],-h5[id|dir|class|align|style],-h6[id|style|dir|class|align|style],hr[class|style],
dd[id|class|title|style|dir|lang],dl[id|class|title|style|dir|lang],dt[id|class|title|style|dir|lang],object[class|id|width|height|codebase|*],
param[name|value|_value|class],embed[type|width|height|src|class|*],map[name|class],area[shape|coords|href|alt|target|class],bdo[class],button[class],iframe[*],
abbr[title|class],q[*],samp[*]]]>
  </validElements>

FYI, I tossed a space in that CDATA element just for the code snippet here (I know of an outstanding Umbraco bug relating to CDATA, so thought I'd avoid that). In the real code, there is no space.

I have no idea why all of the formats work except for "abbr" and "q". Any ideas?