WHATWG

Language

American English.

Dictionary

• behavior
• bitrate
• breadcrumb
• built-in (though see https://github.com/heycam/webidl/issues/350)
• cancelation
• canceled
• canceling
• cipher
• clonable
• color space
• email
• filename
• implementer
• keepalive (though HTTP `Keep-Alive` [sic] header)
• metadata
• nonzero
• overlaid
• popup
• queuing
• referrer (though HTTP `Referer` [sic] header)
• scrollbar
• style sheet
• whitespace (though CSS 'white-space' [sic] property)

Grammar

• Use "'s" for possessives, even when it looks unnatural.
• Use the Oxford Comma.
• Avoid "one of" unless it's followed by a bulleted list. You can normally leave it out and just use "or". If you cannot leave it out, that might be a good indication you want to use a bulleted list for clarity.
• "A hierarchy" (not "an").
• Use e.g., and i.e., instead of their non-comma-ending variants.

Casing

• "web", unless at the start of a sentence

Punctuation

• Spaces around "—" (em dash)
• Lowercase after colon ("The slot attribute is used to assign a slot to an element: an element with a slot attribute is assigned to the slot created by the slot element whose name attribute's value matches that slot attribute's value")

Words and phrases used as words

• When a word or term is not used functionally but is referred to as the word or term itself, enclose it in quotation marks. (See also: use/mention distinction, and some examples that highlight why this is necessary.)

Tone

• Avoid using "simply" or suggesting that something is simple
• Don't use "white" as meaning good and "black" as meaning bad; more concretely, "safelist" and "blocklist" are established precedent