Customize alt text instructions link

app/assets/javascripts/sir_trevor_block_overrides.js

@@ -15,3 +15,8 @@ if (navigator.userAgentData && navigator.userAgentData.brands &&
     Boolean(navigator.userAgentData.brands.find(function(b) { return b.brand === 'Chromium' && parseFloat(b.version, 10) >= 95 && parseFloat(b.version, 10) < 97; }))) {
   SirTrevor.Blocks.Text.prototype.editorHTML = "<div class=\"st-text-block\" spellcheck=\"false\" contenteditable=\"true\"></div>";
 }
+
+// Override the alt text link url so it goes to Stanford's guidelines
+SirTrevor.Locales.en.blocks.alt_text_guidelines = $.extend(SirTrevor.Locales.en.blocks.alt_text_guidelines, {
+  link_url: 'https://uit.stanford.edu/accessibility/concepts/images' 
+});

spec/features/alt_text_instructions_spec.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe 'Alt text instructions', :js do
+  include JavascriptFeatureHelpers
+
+  let(:exhibit) { create(:exhibit) }
+  let(:exhibit_curator) { create(:exhibit_curator, exhibit: exhibit) }
+
+  before do
+    login_as exhibit_curator
+    visit spotlight.edit_exhibit_home_page_path(exhibit)
+    add_widget 'uploaded_items'
+  end
+
+  it 'displays alternative text guidelines with the customized link' do
+    expect(page).to have_content('For each item, please enter alternative text')
+    expect(page).to have_link('Guidelines for writing alt text.', href: 'https://uit.stanford.edu/accessibility/concepts/images')
+  end
+end