Estimating Rendered String Length in XSLT

You can use the i18n library to estimate the rendered length of strings with reasonable accuracy (if you know the font details).

The dci18n:getRenderedTextLength() function takes a string and a font specification (font name, size, and variant) and returns an estimate of its rendered length for that font. It uses the standard Java2D library under the covers. If the font specified is available to Java then it should provide a reasonably accurate estimate of the length, especially for shorter strings. The estimate will not reflect subtleties like kerning or custom inter-word spacing that some layout engines may apply but it's usually accurate to within about one en-width for an 80-character string.

The main challenge is setting the font details correctly. As for all things involving fonts, that usually requires operating-system- and operating-environment-specific configuration. See the Oracle-provided Java2D documentation for details on how to specify fonts. For macOS and Windows it should access installed fonts using the font name shown in the operating system font catalog. For Linux systems the configuration may be different.

For example, to determine the longest term in a set of definition list entries you could do something like this:

...
<!-- Context is a <dl> element -->
<xsl:variable name="terms" as="xs:string*"
  select="normalize-space(*[contains(@class, ' topic/dlentry ']/
                          *[contains(@class, ' topic/dt ']"
/>
<xsl:variable name="maxLength" as="double()"
 select="max(for $t in $terms 
                 return dci18n:getdci18n:getRenderedTextLength($t, 'Arial', '12', 'normal')
        "
/>
...

You can use the rendered length estimator with line breaking to do pretty good layout of text, for example, to generate fixed-layout HTML using absolutely positioned CSS.