That may cause of the <fieldset> has min-inline-size: min-content by default. So we can set min-inline-size: 0 to make width take effect. Example:
html
<fieldset style="width: 10px; background: lightblue">
<pre>xxxxxxxxxxxxxxxxxxxx</pre>
</fieldset>
<fieldset style="min-inline-size: 0; width: 10px; background: lightblue">
<pre>xxxxxxxxxxxxxxxxxxxx</pre>
</fieldset>