How does a element achieve vertical text centering?

Centering Text Vertically in Elements

When using the

element, one may notice that the text inside it is automatically centered vertically. This behavior can be attributed to a combination of factors, including browser-defined styles and specific HTML and CSS aspects.

HTML Structure and Element Hierarchy

Within the

element, Firefox and other browsers create an internal element called moz-button-content. This element is positioned between the tag and its children. Crucially, it is set to display block, preventing CSS from affecting its styling directly.

Browser-Enforced Vertical Positioning

In Firefox, the browser enforces vertical positioning of the button's content. The code responsible for this lives within the source file "nsHTMLButtonControlFrame.cpp." Specifically, it calculates the extra space within the button and adjusts the childPos.B() value accordingly, ensuring that the child content is centered within the available space.

Role of Height in Vertical Centering

Providing a height value for the

enables vertical centering because it overrides the default behavior of the moz-button-content element. Without a specified height, the moz-button-content element will only be as high as its contents, resulting in bottom alignment within the button. By assigning a height, we create additional space within the button, allowing the browser-enforced centering to take effect.

Example

Consider the following example:

<button>Button content</button>

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CSS:

button {
  height: 48px;
}

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In this case, the button's content will be vertically centered within the 48px height, despite the moz-button-content element's display block nature.

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