Documentation cleanup (inc. use of <input ... readonly> for static form controls example)

* Fix incorrect code indentation
* Remove unnecessary vendor prefix for `box-sizing` - all modern browsers now support this unprefixed
* Remove incorrect `<label>` and change static controls to readonly inputs
* Allow `<img>` elements without `src` to allow for `holder.js` images used in the docs, which lack `src` and use `data-src` instead

docs/.htmllintrc

@@ -9,6 +9,7 @@
   "html-valid-content-model": false,
   "id-class-ignore-regex": "(onclick|content|[a-z]+([A-Z][a-z])+)",
   "id-class-style": "dash",
+  "img-req-src": false,
   "img-req-alt": false,
   "indent-style": "spaces",
   "indent-width": 2,