These are all best-practices, rather than absolute musts. Everything listed here is to ensure consistent and coherent writing, but it's quite all right to let your own writer's voice shine through. I am not singling out anyone to make fun of or say that anything you have written is bad. Again, this is mostly for consistency. All of these points are arguable; this would be welcomed, even.
Emphasis in text, whether bold or italic, generally includes containing or following punctuation. For example, in the Backgrounds chapter, the colon following the name of each background in its description is also emboldened.
Sentences have two spaces following them.
Paragraphs have one line between them. The first sentence in a chapter or major section should not be indented, but the remainder should be. This is obviously strongly dependent on the media used (that is, too much of a pain in the ass to deal with on Wikia) and not something to fret over.
Grammar is a bit Canadian. That is, British spelling is preferred, along with North American sentence structure. 'Ass' is preferred over 'arse', and the word 'hospital' takes the definite article.
Colloquial speech is A-okay. Reading the occasional curse word never hurt anyone, so you don't have to watch your damned mouth.
Abbreviations are avoided. Exceptions to this are for acronyms-as-words (the proper term for this escapes me) such as 'laser' or 'scuba'. I.P.B.S. though distinctly a Midian term, falls into this category. Exceptions are also made for common Latin terms known better as acronyms than by the full versions: e.g., i.e., et al. As 'etc.' is fully pronounced--albeit usually incorrectly--we fully write out the phrase 'et cetera' while thumbing our noses at spellcheckers who insist that there not be a space in that phrase.
Contractions are fine, as they represent how speech is used more than a lazy way to write. E.g.: it's, we'll, you'd.
Single quotes are used to stand a word out and indicate that it is not being used in accordance with its definition. We're pointing at a word itself that way. For example, the word 'spud' is used in this example sentence without having anything to do with potatoes. Double quotes, a.k.a. quotation marks, are used for direct quotes.
When introducing a section, new terms used there should be italicised. This includes subsets. That is, in the Backgrounds chapter, the names of those backgrounds used in the introductory paragraphs are in italics.
Names of species and races are always capitalised. There are two reasons for that. The first is scientific convention, for genus names at least. The other is to keep distinct 'humanity' (the quality of kindness and benevolence) and 'Humanity' (the unwashed masses of H. sapiens). Names of attributes most often are. Again, this keeps 'Strength' (the attribute) separate from 'strength' (in its more general and often metaphoric sense).
Capitalisation or italics can be used when describing skills, traits, backgrounds, and other game mechanics. This is not necessary, and unemphasised lowercase usage is preferred. That is, you don't need to say, "Donny knows Lockpicking," when, "Danny knows lockpicking," will suffice, unless it is imperative that it be explicit that you are referencing game mechanics.
I'm terribly lazy. When writing up the Midian guidelines initially, I would put an ampersand instead of the word 'and'. Far, far too many of these bits of Tironian ligatures have survived the early drafts. In preference, going forward and corrected as encountered, ampersands can be used as parts of phrases, but should not be used as a general conjunction.
Skills have the non-descriptive portions centered. The best way to do this in HTML is <div style="text-align: center">Title<br>blah<br>blah<br>blah</div> all on one line. Otherwise, Wikia chokes on new lines being paragraphs, and tries to justify everything except the first and last lines.
Singular titles are preferred on the wiki. It's easy to make something plural, or add another suffix, on a wiki. Put the title in brackets, and add the suffix immediately afterwards, like [[hack\\ing for a page titled 'Hack'. This way, we can link to pages easier within the text. If using the visual editor, it can do this for you.
Instead of carved in stone rules, the use of the word 'guidelines' is preferred.
'Game Master' is preferred over 'Gamemaster', by consensus. I'll freely admit that I've gone back and forth on this issue quite a few times in the past. My current line of thinking is that Gamemaster de-emphasises the 'master' part of the title. 'GM' is right out for official text.