Which attribute is used to define inline CSS styles in an HTML element?

The attribute used to define inline CSS styles in an HTML element is the style attribute. When you use the style attribute, you can apply CSS directly within an HTML tag, allowing for immediate and specific styling of that particular element. This is particularly useful for applying unique styles that are not shared across multiple elements or when you want to override external or internal styles without altering style sheets.

For example, if you have a paragraph element and want to change its text color to red and make it bold, you can do this directly in the HTML code like so:

<p style="color: red; font-weight: bold;">This is a styled paragraph.</p>

This method allows for quick styling, though it's often recommended to use external or internal stylesheets for better maintainability in larger projects. The other attributes such as class and id are used for different purposes: class is used to apply CSS to a group of elements, while id serves to uniquely identify an element, and href is primarily used for hyperlink references.