Formatter introduces visible whitespace in Astro/HTML templates when   is followed by an expression

[loic-bellinger]

Hi Biome team,

I’m running into a formatting behavior that changes the rendered output (visible spacing) in an Astro component.

In HTML, whitespace/newlines between inline nodes can render as a space. In Astro/JSX-like templates, when   is used to prevent wrapping around punctuation (e.g. an en dash), a formatter-introduced newline can become an extra visible space (often "NBSP + normal space").

I know I can avoid this by switching to CSS (white-space: nowrap, or gap etc.) or by restructuring the markup, but I’m raising it because it’s a surprising footgun: formatting "shouldn't" visually change output in this kind of simple inline text case.

Input (before formatting)

<p>
  <time datetime={toISODate(start)}>{startLabel}</time>&nbsp;&ndash;&nbsp;{end ? <time datetime={toISODate(end)}>{endLabel}</time> : endLabel}
</p>

Output (Biome-formatted)

<p>
  <time datetime={toISODate(start)}>{startLabel}</time>&nbsp;&ndash;&nbsp;
  {end ? <time datetime={toISODate(end)}>{endLabel}</time> : endLabel}
</p>

This introduces a newline + indentation whitespace between  –  and {end …}. In the final HTML/rendered output, it produces an extra visible space (effectively   + an additional collapsed space), so the date separator spacing changes.

Formatting should ideally avoid introducing a newline/whitespace text node in a way that changes rendered output, don't you think?