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When using the <abbreviated-form> element in your content, it is possible to style the subsequent occurrences differently than the first occurrence. To achieve this, add something similar to the following rule in your customization CSS:
a:has(dfn[class ~= "abbreviated-form"]) {
    color: oxy_xpath("let $cdf:= dfn return if (preceding::dfn[@keyref = $cdf/@keyref]) then 'black' else 'red'");
    text-decoration: oxy_xpath("let $cdf:= dfn return if (preceding::dfn[@keyref = $cdf/@keyref]) then 'none' else 'underline'");
}

This example would render the first occurrence with a red color and an underline, while the subsequent occurrences would be rendered with a black color and no underline.