CSS Syntax Specification
1. The CSS code is written in the head of HTML and <style></style>includes; or written in a special CSS file.
2. Each attribute in the selector appears as a key-value pair.
3. Remember to mark a semicolon after writing an attribute
CSS code style:
1. It is recommended to use an expanded code style
2. Use lowercase
3. Space specification
a. Before the attribute value, leave a space after the colon
b. Leave a space between the selector and the brace