How to specify font attributes for all elements on an html web page?

* {
 font-size: 100%;
 font-family: Arial;
}

The asterisk implies all elements.

If you're using IE, chances are it will revert to the browser defaults for certain elements, like tables. You can counter that with something like the following CSS:

html, body, form, fieldset, table, tr, td, img {
    margin: 0;
    padding: 0;
    font: 100%/150% calibri,helvetica,sans-serif;
}

input, button, select, textarea, optgroup, option {
    font-family: inherit;
    font-size: inherit;
    font-style: inherit;
    font-weight: inherit;
}

/* rest of your styles; like: */
body {
    font-size: 0.875em;
}

Edit: you may want to read up on CSS resets; see threads like this one