Which CSS property is used to convert text to uppercase or lowercase?

text-transform.

This property controls the case of text in elements:

• none: No change.
• capitalize: The first letter of each word becomes uppercase.
• uppercase: All text becomes uppercase.
• lowercase: All text becomes lowercase.
• full-width: Converts all characters to full-width equivalents (used in Asian languages).

Example usage:

/* All text will be uppercase */
p {
  text-transform: uppercase;
}

/* The first letter of each word will be uppercase */
h1 {
  text-transform: capitalize;
}