Adding multiple CSS classes to an anchor link

If I understand the question, you are asking for a way to add multiple classes in anchor links in Markdown.

As per the README of Goldmark the Markdown processor that is currently turned on by default in Hugo:

Currently only headings support attributes…

## heading ## {#id .className attrName=attrValue class="class1 class2"}

## heading {#id .className attrName=attrValue class="class1 class2"}
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