How Do `` Elements Achieve Vertical Text Alignment Without Explicit `line-height`?

The Enigmatic Vertical Alignment in Elements

In the world of web development, the

element often puzzles us with its seemingly magical ability to keep its text vertically centered. To unravel this mystery, let's analyze a snippet of HTML and CSS:

<code class="html"><button class="button">Some Text</button>
<div class="button">Some Text</div></code>

<code class="css">.button {
  background: darkgrey;
  height: 40px;
  border: 2px solid grey;
  width: 100%;
  box-sizing: border-box;
  font-size: 14px;
  font-family: helvetica;
  text-align: center;
  margin-bottom: 20px;
}</code>

In this scenario, the text in the

element remains centered despite the absence of an explicit line-height property.

Delving into the Browser's Magic

When inspecting the browser's rendering, we uncover a hidden element nested within the

tag. In Firefox, it's called moz-button-content. This element is defined in the browser's user agent stylesheet as:

<code class="css">*|*::-moz-button-content {
  display: block;
}</code>

This hidden element plays a crucial role in vertically centering the text. It's set to display as a block element, causing the element to occupy the entire available height of the button. Within this element, the text is naturally aligned to the center.

The Hard-Coded Vertical Positioning

However, this is not the full story. Browsers employ an additional hard-coded behavior for

elements:

<code class="cpp">// Center child in the block-direction in the button
nscoord extraSpace =
    buttonContentBox.BSize(wm) - contentsDesiredSize.BSize(wm);

childPos.B(wm) = std::max(0, extraSpace / 2);</code>

This code demonstrates that browsers intentionally add extra space on both sides of the text in the moz-button-content element, effectively centering it vertically.

Putting It All Together

In summary, the vertical alignment in

elements arises from two factors:

• The presence of the moz-button-content element, which displays the text as a block and naturally aligns it to the center.
• The hard-coded browser behavior that further centers the moz-button-content element within the button.

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