Final Answer:
The CSS selector to select every element whose href attribute value begins with mailto: is [href^="mailto:"].
Step-by-step explanation:
The CSS attribute selector [href^="mailto:"] is used to select elements based on the value of their href attribute. The ^ (caret) symbol is used to select elements whose href attribute value starts with the specified string. In this case, we are selecting elements whose href attribute value starts with "mailto:".
This selector can be used to style links that open email clients, such as email addresses in a resume or contact page. By styling these links differently, you can provide a better user experience by making it clear that clicking on them will open an email client instead of a web page.