CSS Selectors

Imagine you're at a large party where everyone is wearing name tags. You have party favors to hand out, but they're only for people who meet certain criteria. Maybe you want to give a favor to everyone named "Bob," to those wearing a red shirt, or to those who arrived in a group. How do you quickly find these people? This is essentially what CSS selectors do on a webpage.

CSS selectors help you identify and select specific HTML elements on a webpage so you can style them using CSS. Think of selectors as criteria on name tags that help you decide who gets what party favor. Whether you're targeting all paragraphs, specific class elements, or elements nested in a particular way, CSS selectors are your go-to tools for applying styles.

In this topic, we'll explore the different types of CSS selectors.

Basic Selectors

The basic selectors include:

CSS element selectors use the name of the HTML element to which you want to apply the style as the selector. The styles will be applied to all similar elements in the layout of your web page. Consider the following example:

p {
  color: brown;
}

In the example, the brown color is applied to all HTML elements p on the web page.

CSS id selector is used if you need to work with a specific element when there are many similar elements. It takes its name from the value of the id attribute of the HTML tag you need. A symbol # is placed in front of it so that the browser understands that this is an id selector.

Consider the following css code.

#big {
  font-size: 30px;
}

In this example, the font size is applied only to the HTML tag with the attribute id="big"

CSS class selector is useful when you need to give a lot of different elements the same look. The name of the selector is taken from the value of the class attribute of the desired HTML tag. A dot . is placed in front of it so that the browser understands that this is a class selector.

Here's an example of a CSS code:

.blue {
  background-color: blue;
}

In the example, the background color is applied only to the HTML tags with the attribute class="blue"

The universal CSS selector, denoted as *, allows you to apply specific styles to all elements within an HTML document. This selector is often used for setting a consistent base style across an entire webpage, such as box-sizing, margins, or padding, or even applying fonts.

Here is its syntax:

* {
    /* CSS properties */
}

The universal selector is a useful tool in CSS, but it should be used carefully to ensure that it does not lead to performance issues or conflicts with other styles. It has the lowest specificity, which means any other defined styles will override it.

Specificity

Specificity is a measure used to determine which CSS rule applies if more than one rule matches a particular element. This is calculated based on the different types of selectors used in a CSS rule. The specificity hierarchy from highest to lowest is inline styles, IDs, classes, attributes, and elements. For example, an ID selector (#example) has higher specificity than a class selector (.example), which in turn is more specific than an element selector (div). Specificity is calculated on a per-selector basis, and in cases where two selectors have the same specificity, the latest rule defined in the CSS is applied.

Inheritance

In CSS, inheritance controls how properties flow from parent elements to their children, which helps in avoiding duplication of properties and makes the stylesheet easier to manage. However, not all CSS properties are inheritable. Properties related to text formatting, like font-family, color, and font-size, are typically inherited by child elements, but properties related to box model like width, height, margin, and border are not inherited. This means that unless explicitly specified, child elements do not inherit the box model properties from their parent elements.

Best practices

To write efficient and maintainable CSS:

1. Use class selectors over ID selectors for reusable components: Classes are ideal for styling reusable components because they can be applied to multiple elements without affecting the uniqueness constraint that ID selectors impose. IDs are unique per page and should be used for identifying singular elements that require unique styling or scripting.

2. Keep specificity low to avoid conflicts: Keeping specificity low (for example using single classes or element selectors instead of complex chains or inline styles) ensures styles are easier to override and maintain. This practice reduces the risk of conflicts and makes the debugging process smoother.

3. Use combinators and descendant selectors to minimize redundancy: Use child (>), adjacent sibling (+), and general sibling (~) combinators to target elements in relation to others while writing less code. Descendant selectors (space-separated) help in applying styles to elements nested within others without increasing specificity excessively. This approach promotes cleaner and more scalable CSS.

4. Comment your CSS to explain why specific styles are applied: Comments are crucial for maintaining CSS files, especially in a team environment or for future reference. They should explain why a particular style is applied, any specific considerations or exceptions it covers, and how it interacts with other styles. This practice is particularly useful for unusual or non-obvious styling decisions that could confuse someone revisiting the code.

Conclusion

CSS selectors are a fundamental part of web design, enabling you to apply specific styles to desired elements. Mastering the use of different selectors, understanding specificity and inheritance, and following best practices will enhance your ability to create visually appealing and maintainable websites. As you continue to practice, experiment with these selectors to gain a deeper understanding and proficiency in styling web pages.