Tidying up redundant HTML, CSS & JS docs

Much of the CSS advice was made redundant by various development decisions (and the toolkit).
The advice over usage of HTML5 tags and ARIA needed updating.
Writing JavaScript in [strict mode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope/Strict_mode) is already being applied across many projects and will ensure our code is more robust.