Question: Which statement is true of the single colon (`:`) or double colon (`::`) notations for pseudo-elements-for example, `::before` and `:before`?

1. All browsers support single and double colons for new and older pseudo-elements. So you can use either but it is convention to use single colons for consistency.
2. In CSS3, the double colon notation (`::`) was introduced to create a consistency between pseudo-elements from pseudo-classes. For newer browsers, use the double colon notation. For IE8 and below, using single colon notation (`:`).
3. Only the new CSS3 pseudo-elements require the double colon notation while the CSS2 pseudo-elements do not.
4. In CSS3, the double colon notation (`::`) was introduced to differentiate pseudo-elements from pseudo-classes. However, modern browsers support both formats. Older browsers such as IE8 and below do not.

Answer: The correct answer of the above question is Option D:In CSS3, the double colon notation (`::`) was introduced to differentiate pseudo-elements from pseudo-classes. However, modern browsers support both formats. Older browsers such as IE8 and below do not.