code: public HtmlPolicyBuilder requireRelsOnLinks(String... linkValues) {
      this.invalidateCompiledState();
      if (this.extraRelsForLinks == null) {
        this.extraRelsForLinks = Sets.newLinkedHashSet();
      }
      for (String linkValue : linkValues) {
        linkValue = HtmlLexer.canonicalName(linkValue);
        Preconditions.checkArgument(
            !Strings.containsHtmlSpace(linkValue),
            "spaces in input.  use f(\"foo\", \"bar\") not f(\"foo bar\")");
        this.extraRelsForLinks.add(linkValue);
      }
      if (this.skipRelsForLinks != null) {
        this.skipRelsForLinks.removeAll(this.extraRelsForLinks);
      }
      return this;
    }
docs: Adds
  <a href="https://developer.mozilla.org/en-US/docs/Web/HTML/Link_types"
  >{@code rel="..."}</a> to {@code <a href="...">} tags beyond those in
  {@link #DEFAULT_RELS_ON_TARGETTED_LINKS}.
  <p>
  @see #skipRelsOnLinks