Class attribute of an element may contain more than one class reference. E.g.
<button class="btn btn-primary btn-test">
XPath selector relying on a class must be well formed. For example, the following will not work:
//button[@class='btn-primary']
Correct variant is
//button[contains(concat(' ', normalize-space(@class), ' '), ' btn-primary ')]
btn-primary
class.