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How CSS resolves conflicts: Cascading

In CSS if we have multiple rules, with the same specificity, competing for the same elements then the last declaration always wins.

This is the C, the Cascading, from the CSS acronym.

Cascading rules in the same CSS file

For example, if we have:

p { color: red };
/// other CSS rules
p { color: blue};

Then our paragraphs will be blue.

Cascading rules when importing different CSS files

This is also available when importing CSS files. If the stylesheets have conflicting rules for the same elements, the last imported rule will be applied.

// red.css
p { color: red };
// blue.css
p { color: blue };

And if we import both of these files in the same document

// index.html
<link rel="stylesheet" type="text/css" href="red.css">
<link rel="stylesheet" type="text/css" href="blue.css">

Then our paragraphs will be blue given the order of the files.

It does not matter even if we have the decoration of the CSS in the head of the HTML document. The last imported rule will win.

// index.html
<html>
    <head>
        <style>
            p {color: red;}
        </style>
        <link rel="stylesheet" type="text/css" href="blue.css">
    </head>
    <body>
        <p>This will be a blue paragraph.</p>
    </body>
</html>>

Cascading rules in the same declaration block

Finally, this also applies to the actual declarations blocks of the properties. Take for example the following code:

Like in the previous examples, the paragraphs will still be blue.

p {
    color: red;
    font-size: 16px;
    line-height: 32px;
    background-color: #eee;
    border: none;
    color: blue;
}

By the way, because of situations like this one, it may be a good practice to have the properties declared in alphabetical order and avoid just dropping a new declaration at the end of the block.

p {
    background-color: #eee;
    border: none;
    // conflicts are easyer to track 
    color: red;
    color: blue;
    font-size: 16px;
    line-height: 32px;
}