Why Does CSS2.1 Use `overflow` Other Than 'visible' to Create Block Formatting Contexts?

Why Does CSS2.1 Mandate Overflow Values Other Than "Visible" to Establish a New Block Formatting Context?

In CSS2.1, overflow values other than "visible" are specified to create a new "block formatting context". This decision has raised questions about the rationale behind combining two seemingly unrelated features: the creation of a block formatting context (BFC) and the suppression of overflow.

Historical Context and Key Considerations

According to inquiries made on the CSS mailing list, this decision stems from the need to address scrolling within boxes that intersect with floats. The original CSS 2.0 specification required browsers to rewrap content around intruding floats during scrolling, causing performance issues.

The CSS2.1 amendment prevents this rewrapping by establishing a new BFC for overflow values other than "visible". This ensures that:

• Content within the new BFC is not affected by floats.
• The scrolling behavior of the BFC is unaffected by the presence of floats.

Example

Consider the following example:

<div>...</div>
<div>
  ...:
  ...:
</div>

div {
  height: 80px;
}

div:first-child:before {
  float: left;
  height: 100px;
  margin: 10px;
  content: 'Float';
}

In CSS 2.0, with overflow: visible (default), the second box overlaps the floated element. However, with overflow: auto (CSS 2.1), the second box gains a new BFC, preventing the float from intersecting its content.

Implications of Overflow: Hidden

Even though overflow: hidden typically hides overflowing content, it still allows programmatic scrolling. This is important because boxes with overflow: hidden that contain intersecting floats must also establish a new BFC to prevent rewrapping during scrolling.

Changes Introduced in CSS2.1

CSS2.1 introduces additional changes:

• Block formatting context root elements (elements with overflow values other than "visible") now stretch vertically to the height of their floats.
• Boxes with overflow: hidden and height: auto are restricted by their container's bounds and can no longer intersect floats.

Consistency and Performance

Ultimately, the decision to create a new BFC for overflow values other than "visible" was primarily driven by performance considerations and a desire for consistency in handling overflowing content. This approach optimizes scrolling performance and maintains a predictable layout.

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