From at least Rails 4, the ActionView tag helper turns Array values of HTML options into a single space-separated string Show archive.org snapshot .
This means you can pass an array to :class options:

extra_classes = %w[one two]

= link_to 'Dashboard', root_path, class: ['btn', 'btn-primary', *extra_classes]
=> <a href="/" class="btn btn-primary one two">Dashboad</a>

= content_tag 'div', 'Hello World', class: %w[alert alert-info]
=> <div class="alert alert-info">Hello World</div>

= content_tag 'span', 'Hello World', class: []
=> <span class>Hello World</span>

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