So, I've given a few examples on how to vertically center the text on the page (search the MR threads for a more in-depth discussion). Such techniques work fine and follow the ePub2 rules, but you may have issues with a particular brand of device (ePub on Kobo rings a bell). You can easily modify that technique to put it at the bottom using vertical-align:bottom.
If you are working on an ePub3 book, then it may be a little more elegant to use display:flex rather than the display:table technique.
Code:
CSS:
div.v-btm {height:100vh; width:100vw; display:table; position:fixed;
padding:0; margin:0; text-indent:0}
div.v-btm div {display:table-cell; vertical-align:bottom}
div.v-btm .ctr{text-align:center}
HTML:
<body>
<div class="v-btm">
<div>
<p>Text at the bottom of the page.</p>
<p class="ctr">Text at the bottom of the page and centered.</p>
</div>
</div>
</body>
