Understanding the Difference Between CSS Classes .foo.bar and .foo .bar

Css Css Selectors

The use of classes is a fundamental part of CSS styling. They allow you to target specific HTML elements and apply styles to them. In CSS, you can apply multiple classes to a single HTML element by using a space-separated list of class names in the class attribute. However, there is a significant difference between using classes in the syntax of .foo.bar (without space) and .foo .bar (with space).

1. .foo.bar (without space)

The syntax .foo.bar (without space) is called a compound class selector. It targets elements that have both the classes "foo" and "bar" applied to them, without any other classes in between. This means that an HTML element must have both "foo" and "bar" classes defined in order for the CSS rule to be applied.

For example, consider the following HTML structure:

            
                <div class="foo bar">This element has both "foo" and "bar" classes.</div>

In this case, the CSS rule .foo.bar would target the div element because it has both "foo" and "bar" classes applied to it. If any other class is added in between, such as <div class="foo otherClass bar">, the rule would not apply because the order or presence of other classes would prevent the specific .foo.bar combination.

2. .foo .bar (with space)

The syntax .foo .bar (with space) is called a descendant combinator. It targets elements that have the class "bar" applied to them, but only if they are descendants of elements with the class "foo". This allows you to target specific elements within a particular context.

For example, consider the following HTML structure:

            
                <div class="foo">
                    <div class="bar">This element has the class "bar" and is a descendant of an element with the class "foo".</div>
                </div>

In this case, the CSS rule .foo .bar would target the inner div element because it has the class "bar" and is a descendant of an element with the class "foo". If the inner div element is not a descendant of an element with the class "foo", the rule would not apply.

Usage Examples

Example 1 - Compound Class Selector:

            
                <style>
                    .foo.bar {
                        color: red;
                        font-weight: bold;
                    }
                </style>

                <div class="foo bar">This element has both "foo" and "bar" classes and will have red text color and bold font weight.</div>

Example 2 - Descendant Combinator:

            
                <style>
                    .foo .bar {
                        color: blue;
                        font-style: italic;
                    }
                </style>

                <div class="foo">
                    <div class="bar">This element has the class "bar" and is a descendant of an element with the class "foo". It will have blue text color and italic font style.</div>
                </div>

Conclusion

To summarize the difference between .foo.bar (without space) and .foo .bar (with space), the former targets elements that have both classes applied to them, while the latter targets elements that have the second class applied as a descendant of elements with the first class. Understanding this difference is crucial for effectively styling HTML elements using CSS.

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