Update HTML blog pages

computer-science-blog/pages/css.html

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+<!-- Reading School 2018, HTML page by Matthew Grove, Year 10 -->
+<h2>CSS</h2>
+<p>
+    Cascading Stylesheets (CSS) are used to style websites - i.e. to make them look pretty.
+    Whereas a page without CSS is just displayed as plain content:
+    <br>
+    <img class="presentation-image" src="images/without_css.jpg">
+    <br>
+    One with CSS looks a lot more presentable:
+    <br>
+    <img class="presentation-image" src="images/with_css.jpg">
+    <br>
+    CSS declarations use the <i>tag names</i> (e.g. <code>p</code>), <i>classes</i> and IDs of
+    the elements on the page in order to style them. IDs are written with a preceding hashtag,
+    classes are written with a preceding full stop and tag names are written without any preceding characters.
+    The styles themselves (like <code>height</code>) should all end in semicolons.
+    Here are some example declarations:
+    <br>
+    <code>
+        p {<br>
+            &nbsp;&nbsp;&nbsp;&nbsp;font-size: 3rem;<br>
+        }<br>
+        .class_name {<br>
+            &nbsp;&nbsp;&nbsp;&nbsp;height: 200px;<br>
+        }<br>
+        #id_name {<br>
+            &nbsp;&nbsp;&nbsp;&nbsp;font-family: 'Roboto'<br>
+        }<br>
+    </code>
+    <br>
+    A comment in CSS is started with <code>/*</code> and ended with <code>*/</code>.
+</p>

computer-science-blog/pages/html.html

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+<!-- Reading School 2018, HTML page by Matthew Grove, Year 10 -->
+<h2>HTML</h2>
+<p>
+    Hypertext Markup Language is the standard language used by browsers to display webpages.
+    HTML was invented by Tim Berners-Lee in 1990, and uses tags. This means that in order to define
+    an element (something like a heading or paragraph), you use a <i>tag</i> - such as <code>&lt;p&gt;</code>,
+    which displays a simple paragraph. Most tags must be closed with closing tags - like <code>&lt;/p&gt;</code>,
+    however some elements, like those for images (<code>&lt;img&gt;</code>) and linking external resources
+    (<code>&lt;link&gt;</code>) are auto closing: they don’t need a closing tag. For example, an image is defined as follows:
+    <br>
+    <code>&lt;img src="image_source_url"&gt;</code>
+    <br>
+    HTML is closely linked with CSS and JavaScript; a comment in HTML is started with
+    <code>&lt;!--</code> and ended with <code>--&gt;</code>.
+</p>

computer-science-blog/pages/js.html

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+<!-- Reading School 2018, HTML page by Matthew Grove, Year 10 -->
+<h2>JavaScript</h2>
+<p>
+    JavaScript (JS) is a high-level, client-side language, meaning that it is like a human language
+    (rather than, for example, binary) and runs on the end user's computer. It is used for many purposes,
+    including editing content on a webpage, performing mathematical operations and storing variables.
+    Each line of JavaScript should end in a semicolon, for example:
+    <br>
+    <code>
+        // this code sets the content of the element with the ID "id_name" to "Hello World!"<br>
+        document.getElementById("id_name").innerHTML = "Hello World!";
+    </code>
+    <br>
+    A single line comment in JS is denoted by a double forward slash (<code>//</code>); a multi line comment
+    is started with <code>/*</code> and ended with <code>*/</code>.
+</p>