U
URL
All uppercase, no periods, but URLs themselves (mozilla.org, firefox.com, etc.) are lowercase.
username
Spell as one word, sentence case. Not "user name" or "user-name." For example: “Enter your username and password.”
US
Abbreviation for United States. Use instead of "U.S."
W
web
Lowercase on its own and as part of a compound construction. Capitalize if the word “web” begins a sentence.
webcam
One word, lowercase.
webcast
One word, lowercase.
WebExtensions
Used to describe the API an extension is built on; describes the technology, not an extension built on the technology; see also the WebExtensions wiki entry.
WebFWD
Please note the capitalization, unless dealing with a URL (webfwd.org).
webpage
One word, lowercase.
Web Push
Capitalize when referring to the notification standard.
website
One word, lowercase; do not use "web site."
Wi-Fi
Use Wi-Fi instead of wifi or WiFi.