How to make a back-to-top button using CSS and HTML only?

Utilize the <a> tag.

At the top of your website, put an anchor with specified name.

<a name="top"></a>

Then your "back to top" link points to it.

<a href="#top">back to top</a>

What you would want to do is put an "anchor" at the top of the page, using an <a> tag (it's not JUST useful for links!). Then, when you have a link that goes to #nameofanchor, it scrolls to the anchor with that name. You'd do it like this:

<a id="top"></a>
<!--content here-->
<a href="#top">Back to top</a>

Here is a working jsfiddle: http://jsfiddle.net/qf0m9arp/1/

<a href="#">Start of page</a>

"The link has the href value of "#", which by definition means the start of the current document. Thus there is no need to worry about the correct way of setting up the destination anchor..."

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