ID selectors

The CSS ID selector matches an element based on the value of the element's id attribute. In order for the element to be selected, its id attribute must match exactly the value given in the selector.

/* The element with id="demo" */
#demo {
  border: red 2px solid;
}

#id_value { style properties }

Note that syntactically (but not specificity-wise), this is equivalent to the following attribute selector:

[id=id_value] { style properties }

• CSS Selectors
• Learn CSS: Selectors