app/views/application/_flash.html.erb
<%- flash.each do |key, value| %>
<%= content_tag(:article, class: "main-flash main-flash--#{key}", role: 'alert') do %>
<button type="button" class="close" data-dismiss="alert" aria-label="<%= t(:'.close') %>">
<span aria-hidden="true">×</span>
</button>
<%- Array(value).map do |message| %>
<%= content_tag(:p, message, class: 'main-flash__body') %>
<%- end %>
<%- end %>
<%- end %>