Update HTML blog pages

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>