Applying a CSS Style to a Specific Element with an ID

In CSS, selectors are used to select HTML elements based on their id, class, type, attribute, and other characteristics. When you apply a CSS style to a specific HTML element with a unique id, the best way to do so is with an id selector.

In the given question, the correct answer is #banner { ... }. This code indicates that the CSS styles enclosed within the braces { ... } should be applied to the HTML element with the id 'banner'. This is done through the use of a hash # symbol before the id name. The id attribute in HTML is unique to a particular element and can only be used once on a page. Hence, when the CSS encounters #banner, it knows that the styles should be applied to only the element with that unique 'banner' id.

Let's look at an example:

<div id="banner">This is a banner.</div>

And here is the corresponding CSS:

#banner {
    color: white;
    background-color: black;
    text-align: center;
}

In the above example, the 'banner' id is associated with a div element. The CSS code specifies that the text color within the div should be white, the background color should be black, and the text should be centered.

However, it's important to ensure that you are following best practices when using id selectors. Id selectors are an extremely powerful feature, but be careful not to overuse them. While they create absolutely no issues in terms of performance, taking them to an extreme can make your code hard to manage due to their high specificity.

It’s worth noting that CSS class selectors which are denoted by the dot . (e.g., .banner), are very similar to id selectors. However, instead of applying to just one specific element with that id, class selectors apply styles to all elements with that class. So, if you want to apply the same styles to multiple elements, you should use class selector instead.

In conclusion, the id selector is a fundamental aspect of CSS that allows developers to apply specific styles to unique HTML elements, making it a powerful and essential tool in creating visually compelling and functionally effective web pages.