A footnote reference and footnote text are output to HTML as follows:

Footnote reference:
<sup>[<a name="id394062" href="#ftn.id394062">*</a>]</sup>

Footnote:
<div class="footnote"><p>
<sup>[<a name="ftn.id394062" href="#id394062">*</a>]</sup>
Text of footnote ...

The footnote mark is rendered with superscript sup and square brackets. Each has an href and a name attribute, so they can cross link between each other. If you want to change that rendering, you will need to customize the following two templates from html/footnote.xsl:

Footnote reference template:
<xsl:template match="footnote">
...

Footnote:
<xsl:template match="footnote/para[1]|footnote/simpara[1]" priority="2">
...

You can change the formatting of the footnote paragraph using CSS. Use the div.footnote CSS selector, and apply whatever styles you want with it, as shown in the following example.

div.footnote {
   font-size: 8pt;
}

See the section “Using CSS to style HTML” for more general information about DocBook and CSS.