Suggested changes for HTML chapter

I hope this helps!

Some points:

* I have changed website to webpage in a few places, because here we are clearly talking about just one page
* I have changed deprecated markup such as `<b>` to semantic markup such as `<strong>`, and non-semantic language such as "bigger" to "more important"
* `h1` are headings, not headers (different things!)
* I removed the `<small>` tag - it's [actually for legal information[(http://www.w3.org/TR/html-markup/small.html) (i.e. the "small print") - sorry that it makes the page inconsistent with the image
* I introduced the concept of 'elements' so we are not just talking about tags

I do hope that these changes don't mean extra work changing things later on.