spacer element

<spacer type="horizontal" size="40"> asked the browser for a blank gap of a given size. It was a Netscape extension from the days before CSS, used to push table cells, images and navigation links apart. It still turns up in old templates, usually between links or at the start of a paragraph.

Why avoid

It is obsolete. The HTML Standard lists spacer among the elements that “are entirely obsolete, and must not be used by authors”, with the advice: “Use appropriate elements or CSS instead.”

It also does nothing. The DOM section maps spacer to HTMLUnknownElement, and the rendering section defines no styles for it. The browser ignores its type, size, width, height and align attributes, so the space the author wanted is simply missing from the page, and nothing warns anyone.

It isn’t harmless either, because spacer is not a void element. The spec’s list of void elements, the ones that never have content, is area, base, br, col, embed, hr, img, input, link, meta, source, track and wbr. spacer was almost always written without an end tag, so the parser opens it as a normal element, and everything after it, up to the end of its parent, becomes its content:

<!-- written -->
<p>One<spacer size="40">Two <b>three</b></p>
<!-- parsed -->
<p>One<spacer size="40">Two <b>three</b></spacer></p>

The dead tag ends up wrapping live text. That changes which element is the parent of that content, so child selectors and scripts that walk the DOM see a structure nobody intended.

Use instead

Delete it and put the space in CSS. For a row of items, use gap on a flex or grid container:

<nav class="toolbar">
  <a href="/">Home</a>
  <a href="/about">About</a>
</nav>

.toolbar { display: flex; gap: 2.5rem; }

CSS Box Alignment describes the gap properties as specifying “fixed-length gutters between items in the container”. For space next to one element, use a margin on that element:

.lead { margin-block-end: 1.5rem; }

Detectability

Fully detectable by tag name. There is no autofix: an unclosed spacer contains the content that follows it, so removing the element would delete that content.

Resources

• HTML Standard — Non-conforming features — spacer is entirely obsolete; use CSS instead.
• HTML Standard — Elements in the DOM — spacer maps to HTMLUnknownElement.
• HTML Standard — Void elements — spacer is not void, so an unclosed one wraps the content after it.
• CSS Box Alignment Module Level 3 — gap — fixed-length gutters between items in flex, grid and multi-column containers.