The <noembed> element is a deprecated and non-standard way to provide alternative, or "fallback", content for browsers that do not support the <embed> element or do not support embedded content an author wishes to use. This element was deprecated in HTML 4.01 and above in favor of  <object>. Fallback content should be inserted between the opening and closing <object> tags.
The non-standard SIMD.%type%.toSource() method returns a string representing the source code of the object.
The toSource() method returns a string representing the source code of the object.
The array comprehension syntax is a JavaScript expression which allows you to quickly assemble a new array based on an existing one. Comprehensions exist in many programming languages.
The :-moz-placeholder pseudo-class represents any form element displaying placeholder text. This allows web developers and theme designers to customize the appearance of placeholder text, which is a light grey color by default. This may not work well if you've changed the background color of your form fields to be a similar color, for example, so you can use this pseudo-class to change the placeholder text color.