diff --git a/lib/editor/atto/plugins/accessibilitychecker/yui/build/moodle-atto_accessibilitychecker-button/moodle-atto_accessibilitychecker-button-debug.js b/lib/editor/atto/plugins/accessibilitychecker/yui/build/moodle-atto_accessibilitychecker-button/moodle-atto_accessibilitychecker-button-debug.js
index cdf67fada87..c8d7bb63bce 100644
--- a/lib/editor/atto/plugins/accessibilitychecker/yui/build/moodle-atto_accessibilitychecker-button/moodle-atto_accessibilitychecker-button-debug.js
+++ b/lib/editor/atto/plugins/accessibilitychecker/yui/build/moodle-atto_accessibilitychecker-button/moodle-atto_accessibilitychecker-button-debug.js
@@ -129,8 +129,11 @@ Y.namespace('M.atto_accessibilitychecker').Button = Y.Base.create('button', Y.M.
 
             // Check for non-empty text.
             if (Y.Lang.trim(node.get('text')) !== '') {
-                foreground = node.getComputedStyle('color');
-                background = node.getComputedStyle('backgroundColor');
+                foreground = Y.Color.fromArray(
+                    this._getComputedBackgroundColor(node, node.getComputedStyle('color')),
+                    Y.Color.TYPES.RGBA
+                );
+                background = Y.Color.fromArray(this._getComputedBackgroundColor(node), Y.Color.TYPES.RGBA);
 
                 lum1 = this._getLuminanceFromCssColor(foreground);
                 lum2 = this._getLuminanceFromCssColor(background);
@@ -239,7 +242,7 @@ Y.namespace('M.atto_accessibilitychecker').Button = Y.Base.create('button', Y.M.
      * Generate the HTML that lists the found warnings.
      *
      * @method _addWarnings
-     * @param {Node} A Node to append the html to.
+     * @param {Node} list Node to append the html to.
      * @param {String} description Description of this failure.
      * @param {array} nodes An array of failing nodes.
      * @param {boolean} imagewarnings true if the warnings are related to images, false if text.
@@ -309,6 +312,44 @@ Y.namespace('M.atto_accessibilitychecker').Button = Y.Base.create('button', Y.M.
             b1 = part1(color[2]);
 
         return 0.2126 * r1 + 0.7152 * g1 + 0.0722 * b1;
+    },
+
+    /**
+     * Get the computed RGB converted to full alpha value, considering the node hierarchy.
+     *
+     * @method _getComputedBackgroundColor
+     * @param {Node} node
+     * @param {String} color The initial colour. If not specified, fetches the backgroundColor from the node.
+     * @return {Array} Colour in Array form (RGBA)
+     * @private
+     */
+    _getComputedBackgroundColor: function(node, color) {
+        color = color || node.getComputedStyle('backgroundColor');
+
+        if (color.toLowerCase() === 'transparent') {
+            // Y.Color doesn't handle 'transparent' properly.
+            color = 'rgba(1, 1, 1, 0)';
+        }
+
+        // Convert the colour to its constituent parts in RGBA format, then fetch the alpha.
+        var colorParts = Y.Color.toArray(color);
+        var alpha = colorParts[3];
+
+        if (alpha === 1) {
+            // If the alpha of the background is already 1, then the parent background colour does not change anything.
+            return colorParts;
+        }
+
+        // Fetch the computed background colour of the parent and use it to calculate the RGB of this item.
+        var parentColor = this._getComputedBackgroundColor(node.get('parentNode'));
+        return [
+            // RGB = (alpha * R|G|B) + (1 - alpha * solid parent colour).
+            (1 - alpha) * parentColor[0] + alpha * colorParts[0],
+            (1 - alpha) * parentColor[1] + alpha * colorParts[1],
+            (1 - alpha) * parentColor[2] + alpha * colorParts[2],
+            // We always return a colour with full alpha.
+            1
+        ];
     }
 });
 
diff --git a/lib/editor/atto/plugins/accessibilitychecker/yui/build/moodle-atto_accessibilitychecker-button/moodle-atto_accessibilitychecker-button-min.js b/lib/editor/atto/plugins/accessibilitychecker/yui/build/moodle-atto_accessibilitychecker-button/moodle-atto_accessibilitychecker-button-min.js
index 294089029df..e4a9c3bc743 100644
--- a/lib/editor/atto/plugins/accessibilitychecker/yui/build/moodle-atto_accessibilitychecker-button/moodle-atto_accessibilitychecker-button-min.js
+++ b/lib/editor/atto/plugins/accessibilitychecker/yui/build/moodle-atto_accessibilitychecker-button/moodle-atto_accessibilitychecker-button-min.js
@@ -1 +1 @@
-YUI.add("moodle-atto_accessibilitychecker-button",function(e,t){var n="atto_accessibilitychecker";e.namespace("M.atto_accessibilitychecker").Button=e.Base.create("button",e.M.editor_atto.EditorPlugin,[],{initializer:function(){this.addButton({icon:"e/accessibility_checker",callback:this._displayDialogue})},_displayDialogue:function(){var e=this.getDialogue({headerContent:M.util.get_string("pluginname",n),width:"500px",focusAfterHide:!0});e.set("bodyContent",this._getDialogueContent()).show()},_getDialogueContent:function(){var t=e.Node.create('<div style="word-wrap: break-word;"></div>');return t.append(this._getWarnings()),t.delegate("click",function(e){e.preventDefault();var t=this.get("host"),n=e.currentTarget.getData("sourceNode"),r=this.getDialogue();n?(r.set("focusAfterHide",this.editor).hide(),t.setSelection(t.getSelectionFromNode(n))):r.hide()},"a",this),t},_getWarnings:function(){var t,r=e.Node.create("<div></div>");return t=[],this.editor.all("img").each(function(e){var n=e.getAttribute("alt");(typeof n=="undefined"||n==="")&&e.getAttribute("role")!=="presentation"&&t.push(e)},this),this._addWarnings(r,M.util.get_string("imagesmissingalt",n),t,!0),t=[],this.editor.all("*").each(function(n){var r,i,s,o,u;if(e.Lang.trim(n.get("text"))!==""){r=n.getComputedStyle("color"),i=n.getComputedStyle("backgroundColor"),o=this._getLuminanceFromCssColor(r),u=this._getLuminanceFromCssColor(i),o>u?s=(o+.05)/(u+.05):s=(u+.05)/(o+.05);if(s<=4.5){var a=0,f=!1;for(a=0;a<t.length;a++){if(n.ancestors("*").indexOf(t[a])!==-1){f=!0;break}if(t[a].ancestors("*").indexOf(n)!==-1){t[a]=n,f=!0;break}}f||t.push(n)}}},this),this._addWarnings(r,M.util.get_string("needsmorecontrast",n),t,!1),this.editor.get("text").length>1e3&&!this.editor.one("h3, h4, h5")&&this._addWarnings(r,M.util.get_string("needsmoreheadings",n),[this.editor],!1),t=[],this.editor.all("table").each(function(e){var n=e.one("caption");(n===null||n.get("text").trim()==="")&&t.push(e)},this),this._addWarnings(r,M.util.get_string("tablesmissingcaption",n),t,!1),t=[],this.editor.all("table").each(function(e){var n=e.one("[colspan],[rowspan]");n!==null&&t.push(e)},this),this._addWarnings(r,M.util.get_string("tableswithmergedcells",n),t,!1),t=[],this.editor.all("table").each(function(e){if(e.one("tr").one("td"))e.all("tr").some(function(n){var r=n.one("th");return!r||r.get("text").trim()===""?(t.push(e),!0):!1},this);else{var n=!1;e.one("tr").all("th").some(function(r){return n=!0,r.get("text").trim()===""?(t.push(e),!0):!1}),n||t.push(e)}},this),this._addWarnings(r,M.util.get_string("tablesmissingheaders",n),t,!1),r.hasChildNodes()||r.append("<p>"+M.util.get_string("nowarnings",n)+"</p>"),r},_addWarnings:function(t,r,i,s){var o,u,a,f,l,c,h,p;if(i.length>0){o=e.Node.create("<p>"+r+"</p>"),u=e.Node.create('<ol class="accessibilitywarnings"></ol>'),a=0;for(a=0;a<i.length;a++)c=e.Node.create("<li></li>"),s?(f=i[a].getAttribute("src"),h=e.Node.create('<a href="#"><img src="'+f+'" /> '+f+"</a>")):(l="innerText"in i[a]?"innerText":"textContent",p=i[a].get(l).trim(),p===""&&(p=M.util.get_string("emptytext",n)),i[a]===this.editor&&(p=M.util.get_string("entiredocument",n)),h=e.Node.create('<a href="#">'+p+"</a>")),h.setData("sourceNode",i[a]),c.append(h),u.append(c);o.append(u),t.append(o)}},_getLuminanceFromCssColor:function(t){var n;t==="transparent"&&(t="#ffffff"),n=e.Color.toArray(e.Color.toRGB(t));var r=function(e){return e=parseInt(e,10)/255,e<=.03928?e/=12.92:e=Math.pow((e+.055)/1.055,2.4),e},i=r(n[0]),s=r(n[1]),o=r(n[2]);return.2126*i+.7152*s+.0722*o}})},"@VERSION@",{requires:["color-base","moodle-editor_atto-plugin"]});
+YUI.add("moodle-atto_accessibilitychecker-button",function(e,t){var n="atto_accessibilitychecker";e.namespace("M.atto_accessibilitychecker").Button=e.Base.create("button",e.M.editor_atto.EditorPlugin,[],{initializer:function(){this.addButton({icon:"e/accessibility_checker",callback:this._displayDialogue})},_displayDialogue:function(){var e=this.getDialogue({headerContent:M.util.get_string("pluginname",n),width:"500px",focusAfterHide:!0});e.set("bodyContent",this._getDialogueContent()).show()},_getDialogueContent:function(){var t=e.Node.create('<div style="word-wrap: break-word;"></div>');return t.append(this._getWarnings()),t.delegate("click",function(e){e.preventDefault();var t=this.get("host"),n=e.currentTarget.getData("sourceNode"),r=this.getDialogue();n?(r.set("focusAfterHide",this.editor).hide(),t.setSelection(t.getSelectionFromNode(n))):r.hide()},"a",this),t},_getWarnings:function(){var t,r=e.Node.create("<div></div>");return t=[],this.editor.all("img").each(function(e){var n=e.getAttribute("alt");(typeof n=="undefined"||n==="")&&e.getAttribute("role")!=="presentation"&&t.push(e)},this),this._addWarnings(r,M.util.get_string("imagesmissingalt",n),t,!0),t=[],this.editor.all("*").each(function(n){var r,i,s,o,u;if(e.Lang.trim(n.get("text"))!==""){r=e.Color.fromArray(this._getComputedBackgroundColor(n,n.getComputedStyle("color")),e.Color.TYPES.RGBA),i=e.Color.fromArray(this._getComputedBackgroundColor(n),e.Color.TYPES.RGBA),o=this._getLuminanceFromCssColor(r),u=this._getLuminanceFromCssColor(i),o>u?s=(o+.05)/(u+.05):s=(u+.05)/(o+.05);if(s<=4.5){var a=0,f=!1;for(a=0;a<t.length;a++){if(n.ancestors("*").indexOf(t[a])!==-1){f=!0;break}if(t[a].ancestors("*").indexOf(n)!==-1){t[a]=n,f=!0;break}}f||t.push(n)}}},this),this._addWarnings(r,M.util.get_string("needsmorecontrast",n),t,!1),this.editor.get("text").length>1e3&&!this.editor.one("h3, h4, h5")&&this._addWarnings(r,M.util.get_string("needsmoreheadings",n),[this.editor],!1),t=[],this.editor.all("table").each(function(e){var n=e.one("caption");(n===null||n.get("text").trim()==="")&&t.push(e)},this),this._addWarnings(r,M.util.get_string("tablesmissingcaption",n),t,!1),t=[],this.editor.all("table").each(function(e){var n=e.one("[colspan],[rowspan]");n!==null&&t.push(e)},this),this._addWarnings(r,M.util.get_string("tableswithmergedcells",n),t,!1),t=[],this.editor.all("table").each(function(e){if(e.one("tr").one("td"))e.all("tr").some(function(n){var r=n.one("th");return!r||r.get("text").trim()===""?(t.push(e),!0):!1},this);else{var n=!1;e.one("tr").all("th").some(function(r){return n=!0,r.get("text").trim()===""?(t.push(e),!0):!1}),n||t.push(e)}},this),this._addWarnings(r,M.util.get_string("tablesmissingheaders",n),t,!1),r.hasChildNodes()||r.append("<p>"+M.util.get_string("nowarnings",n)+"</p>"),r},_addWarnings:function(t,r,i,s){var o,u,a,f,l,c,h,p;if(i.length>0){o=e.Node.create("<p>"+r+"</p>"),u=e.Node.create('<ol class="accessibilitywarnings"></ol>'),a=0;for(a=0;a<i.length;a++)c=e.Node.create("<li></li>"),s?(f=i[a].getAttribute("src"),h=e.Node.create('<a href="#"><img src="'+f+'" /> '+f+"</a>")):(l="innerText"in i[a]?"innerText":"textContent",p=i[a].get(l).trim(),p===""&&(p=M.util.get_string("emptytext",n)),i[a]===this.editor&&(p=M.util.get_string("entiredocument",n)),h=e.Node.create('<a href="#">'+p+"</a>")),h.setData("sourceNode",i[a]),c.append(h),u.append(c);o.append(u),t.append(o)}},_getLuminanceFromCssColor:function(t){var n;t==="transparent"&&(t="#ffffff"),n=e.Color.toArray(e.Color.toRGB(t));var r=function(e){return e=parseInt(e,10)/255,e<=.03928?e/=12.92:e=Math.pow((e+.055)/1.055,2.4),e},i=r(n[0]),s=r(n[1]),o=r(n[2]);return.2126*i+.7152*s+.0722*o},_getComputedBackgroundColor:function(t,n){n=n||t.getComputedStyle("backgroundColor"),n.toLowerCase()==="transparent"&&(n="rgba(1, 1, 1, 0)");var r=e.Color.toArray(n),i=r[3];if(i===1)return r;var s=this._getComputedBackgroundColor(t.get("parentNode"));return[(1-i)*s[0]+i*r[0],(1-i)*s[1]+i*r[1],(1-i)*s[2]+i*r[2],1]}})},"@VERSION@",{requires:["color-base","moodle-editor_atto-plugin"]});
diff --git a/lib/editor/atto/plugins/accessibilitychecker/yui/build/moodle-atto_accessibilitychecker-button/moodle-atto_accessibilitychecker-button.js b/lib/editor/atto/plugins/accessibilitychecker/yui/build/moodle-atto_accessibilitychecker-button/moodle-atto_accessibilitychecker-button.js
index 7c149988df8..89d365cda5b 100644
--- a/lib/editor/atto/plugins/accessibilitychecker/yui/build/moodle-atto_accessibilitychecker-button/moodle-atto_accessibilitychecker-button.js
+++ b/lib/editor/atto/plugins/accessibilitychecker/yui/build/moodle-atto_accessibilitychecker-button/moodle-atto_accessibilitychecker-button.js
@@ -129,8 +129,11 @@ Y.namespace('M.atto_accessibilitychecker').Button = Y.Base.create('button', Y.M.
 
             // Check for non-empty text.
             if (Y.Lang.trim(node.get('text')) !== '') {
-                foreground = node.getComputedStyle('color');
-                background = node.getComputedStyle('backgroundColor');
+                foreground = Y.Color.fromArray(
+                    this._getComputedBackgroundColor(node, node.getComputedStyle('color')),
+                    Y.Color.TYPES.RGBA
+                );
+                background = Y.Color.fromArray(this._getComputedBackgroundColor(node), Y.Color.TYPES.RGBA);
 
                 lum1 = this._getLuminanceFromCssColor(foreground);
                 lum2 = this._getLuminanceFromCssColor(background);
@@ -234,7 +237,7 @@ Y.namespace('M.atto_accessibilitychecker').Button = Y.Base.create('button', Y.M.
      * Generate the HTML that lists the found warnings.
      *
      * @method _addWarnings
-     * @param {Node} A Node to append the html to.
+     * @param {Node} list Node to append the html to.
      * @param {String} description Description of this failure.
      * @param {array} nodes An array of failing nodes.
      * @param {boolean} imagewarnings true if the warnings are related to images, false if text.
@@ -304,6 +307,44 @@ Y.namespace('M.atto_accessibilitychecker').Button = Y.Base.create('button', Y.M.
             b1 = part1(color[2]);
 
         return 0.2126 * r1 + 0.7152 * g1 + 0.0722 * b1;
+    },
+
+    /**
+     * Get the computed RGB converted to full alpha value, considering the node hierarchy.
+     *
+     * @method _getComputedBackgroundColor
+     * @param {Node} node
+     * @param {String} color The initial colour. If not specified, fetches the backgroundColor from the node.
+     * @return {Array} Colour in Array form (RGBA)
+     * @private
+     */
+    _getComputedBackgroundColor: function(node, color) {
+        color = color || node.getComputedStyle('backgroundColor');
+
+        if (color.toLowerCase() === 'transparent') {
+            // Y.Color doesn't handle 'transparent' properly.
+            color = 'rgba(1, 1, 1, 0)';
+        }
+
+        // Convert the colour to its constituent parts in RGBA format, then fetch the alpha.
+        var colorParts = Y.Color.toArray(color);
+        var alpha = colorParts[3];
+
+        if (alpha === 1) {
+            // If the alpha of the background is already 1, then the parent background colour does not change anything.
+            return colorParts;
+        }
+
+        // Fetch the computed background colour of the parent and use it to calculate the RGB of this item.
+        var parentColor = this._getComputedBackgroundColor(node.get('parentNode'));
+        return [
+            // RGB = (alpha * R|G|B) + (1 - alpha * solid parent colour).
+            (1 - alpha) * parentColor[0] + alpha * colorParts[0],
+            (1 - alpha) * parentColor[1] + alpha * colorParts[1],
+            (1 - alpha) * parentColor[2] + alpha * colorParts[2],
+            // We always return a colour with full alpha.
+            1
+        ];
     }
 });
 
diff --git a/lib/editor/atto/plugins/accessibilitychecker/yui/src/button/js/button.js b/lib/editor/atto/plugins/accessibilitychecker/yui/src/button/js/button.js
index 1634ac7649a..ce6a4e20212 100644
--- a/lib/editor/atto/plugins/accessibilitychecker/yui/src/button/js/button.js
+++ b/lib/editor/atto/plugins/accessibilitychecker/yui/src/button/js/button.js
@@ -127,8 +127,11 @@ Y.namespace('M.atto_accessibilitychecker').Button = Y.Base.create('button', Y.M.
 
             // Check for non-empty text.
             if (Y.Lang.trim(node.get('text')) !== '') {
-                foreground = node.getComputedStyle('color');
-                background = node.getComputedStyle('backgroundColor');
+                foreground = Y.Color.fromArray(
+                    this._getComputedBackgroundColor(node, node.getComputedStyle('color')),
+                    Y.Color.TYPES.RGBA
+                );
+                background = Y.Color.fromArray(this._getComputedBackgroundColor(node), Y.Color.TYPES.RGBA);
 
                 lum1 = this._getLuminanceFromCssColor(foreground);
                 lum2 = this._getLuminanceFromCssColor(background);
@@ -237,7 +240,7 @@ Y.namespace('M.atto_accessibilitychecker').Button = Y.Base.create('button', Y.M.
      * Generate the HTML that lists the found warnings.
      *
      * @method _addWarnings
-     * @param {Node} A Node to append the html to.
+     * @param {Node} list Node to append the html to.
      * @param {String} description Description of this failure.
      * @param {array} nodes An array of failing nodes.
      * @param {boolean} imagewarnings true if the warnings are related to images, false if text.
@@ -307,5 +310,43 @@ Y.namespace('M.atto_accessibilitychecker').Button = Y.Base.create('button', Y.M.
             b1 = part1(color[2]);
 
         return 0.2126 * r1 + 0.7152 * g1 + 0.0722 * b1;
+    },
+
+    /**
+     * Get the computed RGB converted to full alpha value, considering the node hierarchy.
+     *
+     * @method _getComputedBackgroundColor
+     * @param {Node} node
+     * @param {String} color The initial colour. If not specified, fetches the backgroundColor from the node.
+     * @return {Array} Colour in Array form (RGBA)
+     * @private
+     */
+    _getComputedBackgroundColor: function(node, color) {
+        color = color || node.getComputedStyle('backgroundColor');
+
+        if (color.toLowerCase() === 'transparent') {
+            // Y.Color doesn't handle 'transparent' properly.
+            color = 'rgba(1, 1, 1, 0)';
+        }
+
+        // Convert the colour to its constituent parts in RGBA format, then fetch the alpha.
+        var colorParts = Y.Color.toArray(color);
+        var alpha = colorParts[3];
+
+        if (alpha === 1) {
+            // If the alpha of the background is already 1, then the parent background colour does not change anything.
+            return colorParts;
+        }
+
+        // Fetch the computed background colour of the parent and use it to calculate the RGB of this item.
+        var parentColor = this._getComputedBackgroundColor(node.get('parentNode'));
+        return [
+            // RGB = (alpha * R|G|B) + (1 - alpha * solid parent colour).
+            (1 - alpha) * parentColor[0] + alpha * colorParts[0],
+            (1 - alpha) * parentColor[1] + alpha * colorParts[1],
+            (1 - alpha) * parentColor[2] + alpha * colorParts[2],
+            // We always return a colour with full alpha.
+            1
+        ];
     }
 });