CSS syntax contains set of rules. These rules have 2 parts: a selector, and declaration (property, and a value).
Here, all <h1> elements will be yellow background, and blue text color.
CSS comments are used to explain a block of code or to make temporary changes during development. Comments are ignored by browsers. A comment are single line or multiple lines starts with /* and ends with */.
Case Sensitivity in CSS
The CSS property names and many values are not case-sensitive. But, CSS selectors are usually case-sensitive, for example, the class selector .firstHeading is not the same as .firstheading. Therefore, to be on safer side, you should assume that all components of CSS rules are case-sensitive.