Introduction
CSS selectors are patterns used to select the elements you want to style in your HTML document. They can be used in a CSS stylesheet to apply styles to specific elements on a web page. There are several types of CSS selectors.
Using these different types of selectors, you can create potent and specific styles for your HTML documents.
CSS Declarations Basics
In CSS (Cascading Style Sheets), a declaration is a statement that assigns a value to a property of an element. Declarations are made up of two parts: a property and a value.
The property is the aspect of the element you want to style, and the value is the style you wish to apply to the property.Declarations are written in the following format:
For example, the following declaration sets the color of an element’s text to red:
Declarations are usually grouped together in a declaration block, surrounded by curly braces. The declaration block holds one or more declarations that apply to a specific element or group.Here’s an example of a declaration block:
In this example, the declaration block applies the color, font size, and font-weight properties to all h1 elements on the page.
CSS Selector
Concept Explanation
In CSS, a selector is a pattern that matches elements in a web page. It is used to apply styles to those elements. Several types of selectors include element, class, and id selectors.
Element selectors match elements based on the element’s tag name. For example, the element selector “p” matches all the <p> elements in a web page.
Class selectors match elements based on the element’s class attribute. A class selector begins with a period (.) followed by the class name. For example, the class selector “.warning” matches any element that has a class attribute with the value “warning”.
Id selectors match elements based on the element’s id attribute. An id selector begins with a hash symbol (#) followed by the id value. For example, the id selector “#main” matches the element with an id attribute of “main”.
Here’s a sample of how you might use these selectors in a stylesheet:
In this example, all <p> elements will be rendered in red, any element with a class of “warning” will have a yellow background, and the element with an id of “main” will have a font size of 18 pixels.
Universal CSS Selector
The universal selector is denoted by the asterisk (*) character and matches any element on the page. It can be used to apply styles to all elements on the page, or it can be combined with other selectors to select a subset of elements.
For example, the following CSS rule uses the universal selector to apply a style to all elements on the page:
This rule will make all the text on the page red.You can also use the universal selector in combination with other selectors to select a specific subset of elements. For example:
This rule will apply the font-weight: bold style to all p elements on the page.It’s important to note that the universal selector has a very high specificity, meaning that it will override any other styles that have been applied to an element unless those styles have an even higher specificity.
This can be useful in some instances, but it’s generally a good idea to use more specific selectors to avoid unintended consequences.
CSS Class Selector
A class selector is a CSS selector used to select elements with a specific class attribute. To use a class selector, you must first define the class in your HTML code by using the class attribute.
Then, in your CSS code, you can use the class selector by prefixing the class name with a period (.).Here’s an example of how you might use a class selector in your CSS code.
In this example, the class selector .warning is used to select all elements with a class attribute of “warning” and apply the specified styles (in this case, setting the text color to red and the font weight to bold).
To use the class selector in your HTML code, you would apply the class to an element by including the class attribute and the class name:
In this example, the p element with the “warning” class will be styled according to the styles defined in the CSS class selector.
CSS Multiple Classes
You can apply styles to an element in CSS by specifying its class. If an element has multiple classes, you can specify all of the classes in the element’s class attribute, separated by spaces.For example, given the following HTML:
You can apply styles to the div element by specifying the classes in your CSS:
You can also combine multiple classes in a single CSS rule. For example:
This will apply the red and large styles to both classes’ elements.You can also use the: not() pseudo-class to specify styles for elements that do not have a particular class. For example:
This will apply a black border to all .box elements that do not have the red class.
CSS ID Selector Explained
In CSS, the ID selector allows you to select an element on a page based on its unique ID. The ID of an element is defined by the id attribute in the HTML code.
To use the ID selector, you include a # symbol followed by the ID of the element you want to select. For example:
This will select the element with the ID myElement and change its text color to red.Here is an example of how you might use the ID selector in your HTML code:
It’s important to note that an ID must be unique within a page. You cannot use the same ID for multiple elements on the same page.
You can also use the ID selector in combination with other selectors to create more specific rules. For example:
This will select all p elements inside the element with the ID myElement and change their text color to blue.
CSS Attribute Selector
In CSS, an attribute selector is a pattern used to select elements based on their attributes and attribute values. It is used to determine elements with a specific attribute or attribute with a particular value.
Here is an example of how to use an attribute selector to select all elements with a specific attribute:
You can also use an attribute selector to select elements with a specific attribute value. For example, the following attribute selector will select all elements with a “title” attribute that has the value “example”:
You can also use the *= operator to select elements with an attribute value that contains a specific word. For example, the following attribute selector will select all elements with a “title” attribute that contains the word “example”:
There are many other ways to use attribute selectors, such as selecting elements with an attribute value that starts with a specific word, ends with one particular word, or is a space-separated list of words.
You can find more information about attribute selectors in the CSS specification.
CSS Pseudo-Class Selector
In Cascading Style Sheets (CSS), a pseudo-class is a keyword added to a selector that specifies a particular state of the selected element(s).
For example, the :hover pseudo-class applies styles to an element when the user hovers over it with a mouse.Here is an example of how you can use a pseudo-class in your CSS:
In the above example, the a:hover pseudo-class selects all a elements that are being hovered over by the mouse, and the color property is set to red.
There are several other pseudo-classes that you can use in CSS, including :active, :focus, :first-child, :last-child, and :nth-child. You can also combine pseudo-classes with other selectors to create more specific styles.
For example, you can use a :hover img to select all images within the elements being hovered over.
It’s important to note that pseudo-classes differ from pseudo-elements, which are used to style specific parts of an element, rather than the element itself.
CSS Specificity selector
In CSS, specificity is a way of determining which style rules should be applied to an element. When multiple style rules could potentially apply to the same element, the browser determines which one to use based on the selector’s specificity.
Four factors contribute to the specificity of a selector:Type selectors (e.g., p, div, h1) and pseudo-elements (e.g., ::before, ::after) have a lower specificity than classes, IDs, and attributes.Class selectors (e.g., .highlight), attribute selectors (e.g., [type=”text”]), and pseudo-classes (e.g., :hover, :active) have a higher specificity than type selectors but a lower specificity than IDs.
IDs (e.g., #header) have the highest specificity of all.Inline styles (i.e., styles applied directly to an element using the style attribute) have the highest specificity, overriding any other styles that might apply to the element.For example, consider the following style rules:
If we apply these rules to the following HTML:
The p element would be blue because the p rule has a lower specificity than the .highlight and #header rules, which do not apply to the p element. The div element would be red because the #header rule has the highest specificity and applies to the div element.
It’s important to note that specificity is only used to determine which style rule to apply when conflict arises. All style rules will be applied to the element if there is no conflict.
You can use the !important keyword to prioritize a style rule over all other rules, regardless of specificity. However, it’s generally considered a best practice to avoid using !important whenever possible, as it can make your stylesheets more challenging to maintain.
CSS Chaining selector
In CSS, you can use the chaining (also known as descendant) selector to select elements that are descendants of a specific element. This is useful when you want to style elements nested within other elements.
Here’s an example of using the chaining selector:
In this example, the chaining selector div p will select all p elements that are descendants of div elements and apply the font-size and color styles.
You can chain multiple selectors together to create more specific rules. For example:
This will select only p elements that are descendants of div elements with the class my-class.It’s important to note that the chaining selector will only select elements that are descendants of the specified element, not just elements nested within it.
For example, consider the following HTML:
If you use the chaining selector div p, it will select the p element because it is a descendant of a div element. However, if you use the chaining selector .inner p, it will not select the p element because it is not a descendant of an element with the class inner.
Descendant Combinator selector
In CSS, the descendant combinator selector is used to select elements that are descendants of a specific element. It is represented by a single space between two elements.For example, consider the following HTML:
To select the span element that is a descendant of the div with the class container, you could use the following CSS:
This will apply the style rules to the span element because it is a descendant of the div element with the class container.
Note that the descendant combinator selector will match elements that are descendants at any level, not just direct children. So in the example above, the span element is a descendant of the div with the class container, even though it is not a direct child.
It is also possible to chain multiple descendant combinator selectors to select elements that are descendants of various elements. For example, the following CSS will select any p element that is a descendant of a div element with the class container:
CSS Multiple Selectors
In CSS, you can use multiple selectors to apply the same style rules to numerous elements simultaneously. There are several ways to specify numerous selectors:Separate each selector with a comma:
This will apply red to all h1, h2, and h3 elements.Use descendant selectors:
This will apply the color blue to all p elements descendants of a body element.Use the :not() pseudo-class:
This will apply the color green to all elements that are not h1 elements.Use attribute selectors:
This will apply a 1px solid black border to all input elements with a type attribute value of “text”.You can combine multiple selectors in various ways to create more specific styles. For example:
This will apply the color red to all p elements with a class of “warning” that are descendants of a body element.
Conclusion
A CSS selector is a pattern used to select elements in an HTML document to apply styles to them. There are various types of CSS selectors, including element, class, ID, attribute, and pseudo-class selectors.
Element selectors allow you to select elements based on their tag name, such as p, div, or h1. Class selectors allow you to select elements based on their class attribute, using a . followed by the class name.
ID selectors allow you to select elements based on their ID attribute, using a # followed by the ID value. Attribute selectors permit you to select elements based on the values of their attributes using square brackets and attribute conditions.
Pseudo-class selectors allow you to select elements based on their state or position, such as :hover, :active, or :first-child.
CSS selectors can be combined to create more specific and complex selections. For example, you can use a combination of element, class, and attribute selectors to select a particular element with a specific class and attribute value.
CSS selectors are an essential part of CSS and are used to apply styles to elements in an HTML document. Understanding how to use CSS selectors effectively is an essential skill for web developers and designers.
