Why Does My Input Element Overflow its Parent When I Use `width: 100%`?

Understanding the Issue with CSS Input Bounds

In web development, it is common to encounter situations where HTML input elements exceed their parent's boundaries when width: 100% is applied. This issue arises due to the CSS box model, which explains how elements are sized and laid out.

The Box Model

The CSS box model considers an element's dimensions in four parts:

• Content Box: The innermost area containing the element's content, such as text or images.
• Padding: The space between the content box and the element's border.
• Border: The line surrounding the element.
• Margin: The space outside the border.

Impact of Padding on Dimensions

By default, the width and height properties of an element are applied to its content box. However, when padding is present, it extends beyond the content box, increasing the overall size of the element.

If you set width: 100% on an element with padding, the padding will push the element's width beyond 100% of its parent's width, causing it to exceed the boundaries.

Adjusting the Behavior with box-sizing

To prevent padding from affecting the element's width, you can set the box-sizing property to border-box. This tells the browser to include the padding within the element's width and height calculations.

Here's the CSS code using box-sizing:

input[type=text],
input[type=password] {
  width: 100%;
  box-sizing: border-box;
}

Best Practice

Consider the following best practice for consistent sizing:

html {
  box-sizing: border-box;
}
*,
*:before,
*:after {
  box-sizing: inherit;
}

This ensures that border-box sizing is inherited by all elements, avoiding potential inconsistencies.

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