In CSS syntax, how should property names be formatted?

In CSS syntax, property names are universally formatted in lower case and are typically separated by hyphens. This convention is crucial for the proper functioning of stylesheets, as the CSS specification dictates that property names must be case-sensitive and can only consist of specific characters, including letters, numbers, and hyphens.

Using this format not only adheres to the CSS standards but also ensures compatibility across different browsers and rendering engines. For instance, the correct way to define a property such as "background color" is to write it as "background-color". This approach avoids confusion and potential errors that could arise from using inconsistent capitalization or incorrect separators.

The other options present formatting styles that do not align with CSS syntax rules, leading to invalid property names that would be ignored by the browser or cause rendering issues. Hence, understanding and applying the correct formatting of CSS property names is essential for web development and design.