Understanding the ID Selector in CSS

The ID selector is used in CSS (Cascading Style Sheets) to specify styles for a single, unique HTML element. The correct answer to the question "Which selector is used to specify a style for one unique element?" is therefore "id".

To use the ID selector in CSS, you start with a hash symbol (#) followed by the ID name. An example of this would be #header or #footer. This ID can then be used to apply specific styles to that single element.

Practical Examples

Let's consider a practical example. You may choose to style a unique navigation bar at the top of your webpage. Suppose you assign this element an ID called "topnav". To style this unique element, your CSS would look something like this:

#topnav {
  background-color: #333;
  color: white;
  padding: 14px 16px;
}

This style declaration targets only the HTML element with the ID "topnav", making the background color dark gray (as indicated by the hexadecimal code #333), the text color white, and adding padding of 14px top and bottom,16px left and right.

Best Practices and Additional Insights

While ID selectors are powerful tools in CSS, it's also crucial to use them correctly and judiciously.

1. Uniqueness: Each ID should be unique within a page. No two elements should have the same ID. If you want to style several elements in the same way, it's better to use a class selector.

2. Specificity: The ID selector has a high specificity value, meaning that if you use an ID selector and a class selector to style an element, the ID selector's style will override the class selector's style due to its higher specificity.

3. JavaScript: In addition to CSS, IDs can also be useful in JavaScript for targeting specific elements.

Remember that ID selectors are designed for unique elements used once per page, whereas class selectors are best for styling that can be reused across multiple elements. Following these best practices ensures your styles are targeted, efficient, and easy to manage.