diff --git a/tests/javascripts/highlightTags.test.js b/tests/javascripts/highlightTags.test.js
new file mode 100644
index 000000000..cadac0b9b
--- /dev/null
+++ b/tests/javascripts/highlightTags.test.js
@@ -0,0 +1,449 @@
+const helpers = require('./support/helpers.js');
+
+beforeAll(() => {
+  require('../../app/assets/javascripts/highlightTags.js');
+});
+
+afterAll(() => {
+  require('./support/teardown.js');
+});
+
+describe('Highlight tags', () => {
+
+  let input;
+  let textarea;
+  let backgroundEl;
+
+  beforeAll(() => {
+
+    // set some default styling
+    const stylesheet = document.createElement('style');
+
+    stylesheet.innerHTML = ".textbox-highlight-textbox { padding: 2px; width: 576px; border-width: 1px; }";
+    stylesheet.innerHTML += "textarea.textbox-highlight-textbox { height: 224px; }";
+
+    document.getElementsByTagName('head')[0].appendChild(stylesheet);
+
+  });
+
+  afterAll(() => {
+
+    stylesheet.parentNode.removeChild(stylesheet);
+
+  });
+
+  beforeEach(() => {
+
+    // set up DOM
+    document.body.innerHTML = `
+      <div class="form-group">
+        <label for="subject">Subject</label>
+        <input class="form-control textbox-highlight-textbox" data-module="highlight-tags" type="text" name="subject" id="subject" />
+      </div>
+      <div class="form-group">
+        <label for="template_content">Message</label>
+        <textarea class="form-control form-control-1-1 textbox-highlight-textbox" data-module="highlight-tags" id="template_content" name="template_content" rows="8">
+        </textarea>
+      </div>`;
+
+    input = document.querySelector('input');
+    textarea = document.querySelector('textarea');
+
+  });
+
+  afterEach(() => {
+
+    document.body.innerHTML = '';
+
+  });
+
+  describe("When the page loads", () => {
+
+    describe("An element should be added as a layer below the textbox to hold the highlights", () => {
+
+      beforeEach(() => {
+
+        // start module
+        window.GOVUK.modules.start();
+
+      });
+
+      test("If the textbox is a <textarea>", () => {
+
+        backgroundEl = textarea.nextElementSibling;
+
+        // both the textbox and the element behind need a wrapping element
+        expect(textarea.parentNode.classList.contains('textbox-highlight-wrapper')).toBe(true);
+
+        expect(backgroundEl).not.toBeNull();
+        expect(backgroundEl.classList.contains('textbox-highlight-background')).toBe(true);
+
+      });
+
+      test("If the textbox is an <input>", () => {
+
+        backgroundEl = input.nextElementSibling;
+
+        // both the textbox and the element behind need a wrapping element
+        expect(input.parentNode.classList.contains('textbox-highlight-wrapper')).toBe(true);
+
+        expect(backgroundEl).not.toBeNull();
+        expect(backgroundEl.classList.contains('textbox-highlight-background')).toBe(true);
+
+      });
+
+    });
+
+    describe("The element's dimensions and border-width should match those of the textbox", () => {
+
+      beforeEach(() => {
+
+        // start module
+        window.GOVUK.modules.start();
+
+      });
+
+      test("If the textbox is an <textarea>", () => {
+
+        backgroundEl = textarea.nextElementSibling;
+
+        // element has box-sizing: border-box so width includes padding and border
+        expect(backgroundEl.style.width).toEqual('582px');
+        expect(backgroundEl.style.borderWidth).toEqual('1px');
+
+      });
+
+      test("If the textbox is an <input>", () => {
+
+        backgroundEl = input.nextElementSibling;
+
+        // element has box-sizing: border-box so width includes padding and border
+        expect(backgroundEl.style.width).toEqual('582px');
+        expect(backgroundEl.style.borderWidth).toEqual('1px');
+
+      });
+
+    });
+
+    test("The element should be hidden from assistive technologies", () => {
+
+      expect(backgroundEl.getAttribute('aria-hidden')).toEqual('true');
+
+    });
+
+    describe("If there is a variable in the content, its matching text in the element below should be wrapped in a highlight tag", () => {
+
+      test("If the textbox is a <textarea>", () => {
+
+        textarea.textContent  = "Dear ((title)) ((name))";
+
+        // start module
+        window.GOVUK.modules.start();
+
+        backgroundEl = textarea.nextElementSibling;
+
+        const highlightTags = backgroundEl.querySelectorAll('.placeholder');
+
+        expect(highlightTags.length).toEqual(2);
+        expect(highlightTags[0].textContent).toEqual('((title))');
+        expect(highlightTags[1].textContent).toEqual('((name))');
+
+      });
+
+      test("If the textbox is a <input>", () => {
+
+        input.value = "Dear ((title)) ((name))";
+
+        // start module
+        window.GOVUK.modules.start();
+
+        backgroundEl = input.nextElementSibling;
+
+        const highlightTags = backgroundEl.querySelectorAll('.placeholder');
+
+        expect(highlightTags.length).toEqual(2);
+        expect(highlightTags[0].textContent).toEqual('((title))');
+        expect(highlightTags[1].textContent).toEqual('((name))');
+
+      });
+
+    });
+
+    describe("If there is optional text in the content, its matching text in the element below should be wrapped in a highlight tag", () => {
+
+      test("If the textbox is a <textarea>", () => {
+
+        textarea.textContent = "When you arrive, please go to the ((weekday??main entrance))((weekend??side entrance))";
+
+        // start module
+        window.GOVUK.modules.start();
+
+        backgroundEl = textarea.nextElementSibling;
+
+        const highlightTags = backgroundEl.querySelectorAll('.placeholder-conditional');
+
+        expect(highlightTags.length).toEqual(2);
+        expect(highlightTags[0].textContent).toEqual('((weekday??');
+        expect(highlightTags[1].textContent).toEqual('((weekend??');
+
+      });
+
+      test("If the textbox is an <input>", () => {
+
+        input.value = "When you arrive, please go to the ((weekday??main entrance))((weekend??side entrance))";
+
+        // start module
+        window.GOVUK.modules.start();
+
+        backgroundEl = input.nextElementSibling;
+
+        const highlightTags = backgroundEl.querySelectorAll('.placeholder-conditional');
+
+        expect(highlightTags.length).toEqual(2);
+        expect(highlightTags[0].textContent).toEqual('((weekday??');
+        expect(highlightTags[1].textContent).toEqual('((weekend??');
+
+      });
+
+    });
+
+  });
+
+  describe("When the content of the textbox is updated", () => {
+
+    // doesn't apply to inputs as they have a fixed height
+    test("If new input changes the textarea's dimensions, the size of the element below should still match", () => {
+
+      // set 10 lines of content
+      textarea.textContent = `
+      Ref: ((reference))
+      Date: ((date))
+      NHS number: ((nhs_number))
+
+      Dear ((name))
+
+      Thank you for attending the appointment on ((appointment_date)).
+
+      We will now pass on the results to your GP, ((doctor)), who will be in contact with you soon to arrange a follow up appointment.
+
+      `;
+
+      // start module
+      window.GOVUK.modules.start();
+
+      backgroundEl = textarea.nextElementSibling;
+
+      // add another line of text to the content
+      textarea.textContent += "Best Regards\n\n((requester))";
+
+      // mock calls for the background element's current height
+      jest.spyOn(backgroundEl, 'offsetHeight', 'get').mockImplementation(() => 248);
+
+      helpers.triggerEvent(textarea, 'input');
+
+      expect(window.getComputedStyle(textarea).height).toEqual("248px");
+
+    });
+
+    describe("If a new variable is added to the textbox, it should also be added to the element below in a highlight tag", () => {
+
+      test("If the textbox is a <textarea>", () => {
+
+        textarea.textContent = "Dear ((title)) ((name))";
+
+        // start module
+        window.GOVUK.modules.start();
+
+        backgroundEl = textarea.nextElementSibling;
+
+        // add some more content with a new variable
+        textarea.textContent += "\nRef: ((reference))";
+        helpers.triggerEvent(textarea, 'input');
+
+        const highlightTags = backgroundEl.querySelectorAll('.placeholder');
+
+        expect(highlightTags.length).toEqual(3);
+        expect(highlightTags[2].textContent).toEqual('((reference))');
+
+      });
+
+      test("If the textbox is an <input>", () => {
+
+        input.value = "Hospital appointment for ((name))";
+
+        // start module
+        window.GOVUK.modules.start();
+
+        backgroundEl = input.nextElementSibling;
+
+        // add some more content with a new variable
+        input.value += ", ref: ((reference))";
+        helpers.triggerEvent(input, 'input');
+
+        const highlightTags = backgroundEl.querySelectorAll('.placeholder');
+
+        expect(highlightTags.length).toEqual(2);
+        expect(highlightTags[0].textContent).toEqual('((name))');
+        expect(highlightTags[1].textContent).toEqual('((reference))');
+
+      });
+
+    });
+
+    describe("If a new piece of optional text is added to the textbox, it should also be added to the element below in a highlight tag", () => {
+
+      test("If the textbox is a <textarea>", () => {
+
+        textarea.textContent = "Dear ((title)) ((name))";
+
+        // start module
+        window.GOVUK.modules.start();
+
+        backgroundEl = textarea.nextElementSibling;
+
+        // add some more content with some optional content inside
+        textarea.textContent += "\nYour appointment will be on ((date)). When you arrive, please go to the ((weekday??main entrance))((weekend??side entrance))";
+        helpers.triggerEvent(textarea, 'input');
+
+        const highlightTags = backgroundEl.querySelectorAll('.placeholder');
+        const optionalHighlightTags = backgroundEl.querySelectorAll('.placeholder-conditional');
+
+        expect(highlightTags.length).toEqual(3);
+        expect(optionalHighlightTags.length).toEqual(2);
+        expect(optionalHighlightTags[0].textContent).toEqual('((weekday??');
+        expect(optionalHighlightTags[1].textContent).toEqual('((weekend??');
+
+      });
+
+      test("If the textbox is an <input>", () => {
+
+        input.value = "Hospital appointment";
+
+        // start module
+        window.GOVUK.modules.start();
+
+        backgroundEl = input.nextElementSibling;
+
+        // add some more content with some optional content inside
+        input.value += "((important?? - IMPORTANT))";
+        helpers.triggerEvent(input, 'input');
+
+        const optionalHighlightTags = backgroundEl.querySelectorAll('.placeholder-conditional');
+
+        expect(optionalHighlightTags.length).toEqual(1);
+        expect(optionalHighlightTags[0].textContent).toEqual('((important??');
+
+      });
+
+    });
+
+    describe("If a variable is removed from the textbox, its highlight should also be removed", () => {
+
+      test("If the textbox is a <textarea>", () => {
+
+        textarea.textContent = `
+          Dear ((title)) ((name))
+
+          Ref: ((reference))`;
+
+        // start module
+        window.GOVUK.modules.start();
+
+        backgroundEl = textarea.nextElementSibling;
+
+        // add some more content with a new variable
+        textarea.textContent = "Dear ((title)) ((name))";
+        helpers.triggerEvent(textarea, 'input');
+
+        const highlightTags = backgroundEl.querySelectorAll('.placeholder');
+
+        expect(highlightTags.length).toEqual(2);
+        expect(highlightTags[0].textContent).toEqual('((title))');
+        expect(highlightTags[1].textContent).toEqual('((name))');
+
+      });
+
+      test("If the textbox is an <input>", () => {
+
+        input.value = "Hospital appointment for ((name)), ref: ((reference))";
+
+        // start module
+        window.GOVUK.modules.start();
+
+        backgroundEl = input.nextElementSibling;
+
+        // add some more content with a new variable
+        input.value = "Hospital appointment for ((name))";
+        helpers.triggerEvent(input, 'input');
+
+        const highlightTags = backgroundEl.querySelectorAll('.placeholder');
+
+        expect(highlightTags.length).toEqual(1);
+        expect(highlightTags[0].textContent).toEqual('((name))');
+
+      });
+
+    });
+
+    describe("If a piece of optional text has been removed from the textbox, its highlight should also be removed", () => {
+
+      test("If the textbox is a <textarea>", () => {
+
+        textarea.textContent = `
+          Dear ((title)) ((name))
+
+          Your appointment will be on ((date)). When you arrive, please go to the ((weekday??main entrance))((weekend??side entrance)).`;
+
+        // start module
+        window.GOVUK.modules.start();
+
+        backgroundEl = textarea.nextElementSibling;
+
+        // add some more content with a new variable
+        textarea.textContent = `
+          Dear ((title)) ((name))
+
+          Your appointment will be on ((date)). When you arrive, please go to the ((weekday??main entrance))`;
+
+        helpers.triggerEvent(textarea, 'input');
+
+        const highlightTags = backgroundEl.querySelectorAll('.placeholder');
+        const optionalHighlightTags = backgroundEl.querySelectorAll('.placeholder-conditional');
+
+        expect(highlightTags.length).toEqual(3);
+        expect(optionalHighlightTags.length).toEqual(1);
+        expect(highlightTags[0].textContent).toEqual('((title))');
+        expect(highlightTags[1].textContent).toEqual('((name))');
+        expect(optionalHighlightTags[0].textContent).toEqual('((weekday??');
+        expect(highlightTags[2].textContent).toEqual('((date))');
+
+      });
+
+      test("If the textbox is an <input>", () => {
+
+        input.value = "Hospital appointment for ((name))((important?? - IMPORTANT))";
+
+        // start module
+        window.GOVUK.modules.start();
+
+        backgroundEl = input.nextElementSibling;
+
+        // add some more content with a new variable
+        input.value = "Hospital appointment for ((name))"
+
+        helpers.triggerEvent(input, 'input');
+
+        const highlightTags = backgroundEl.querySelectorAll('.placeholder');
+        const optionalHighlightTags = backgroundEl.querySelectorAll('.placeholder-conditional');
+
+        expect(highlightTags.length).toEqual(1);
+        expect(optionalHighlightTags.length).toEqual(0);
+        expect(highlightTags[0].textContent).toEqual('((name))');
+
+      });
+
+    });
+
+  });
+
+});