Why does CSS padding increase size of an element?

Explaining the box-sizing property

When we define the width and height of an element, we're actually only setting the size of the content box by default. The total rendered size of the element also includes the padding, border, and potentially the margin. The box-sizing: border-box; property toggles the box model so that the dimensions, width and height, involves the padding and borders. This ensures the design of the webpage remains the way you intended under the influence of padding; no sudden layout altercations!

Delving into the box-sizing values

Get to know the border-box value

Border-box is a value of the box-sizing property that allows width and height to include padding and border, but not the margin. With this, the elusive padding no longer adds to the overall size of the element. Margins still affect the element's positioning but not the predetermined dimensions.

Assuring a consistent cross-browser experience

Be it Chrome, Safari or Firefox, consistency among browsers is crucial. Understandably, before CSS3, browsers had their own representations for box-sizing. But with the advent of box-sizing, it’s become practical to shape uniform layouts.

.element {
   -webkit-box-sizing: border-box; /* Safari/Chrome, other WebKit */
   -moz-box-sizing: border-box;    /* Firefox, other Gecko */
   box-sizing: border-box;         /* Opera/IE 8+ */
} 


Never, ever forget to include appropriate vendor prefixes to your box-sizing property, especially for full compatibility.

Precise total width management

In an ideal world, the width, padding, borders, and margins of your elements should not exceed 100%. If you have to allot a total 100% width for an element with padding, the padding must be taken into account too.

.container {
   width: calc(100% - 20px); /* Exclude 10px of padding on each side */
   padding: 10px;
}


But with border-box, you can set width to 100% and not worry about excluding padding. The padding and borders will be taken into consideration, inside this width!

.container {
    box-sizing: border-box;
    width: 100%; /* padding and borders will be inside */
    padding: 10px;
}


Wrapping components aptly

Choosing alt methods when padding doesn't fit the bill

There are certain times when resulting to padding might not be beneficial. You can alternatively encase your content within a <div> with margins. This forms a layer of "pseudo-padding" without altering the size of the element. You could also conduct fixed and automatic width management for wrapper tags to bolster padding and sizing control.

Enhancing user experience via CSS box model understanding

Understanding firmness of the CSS box model directly influences user experience. It capacitates the creation of adaptable layouts that are accessible on any device. A well-constructed design and layout are more essential than some may think, and padding plays a significant part in that.