Fonts display poorly in Pages for iCloud Beta on Safari, but OK in Firefox and Chrome

Does anyone know why fonts display poorly in Pages for iCloud Beta when on Safari, but everything looks perfectly fine in Firefox and Chrome? In all cases, the latest versions of the browsers with no extensions in use and on the same MacBook Air running Yosemite (and the same problem occurs using the same software on a Mac Mini also running Yosemite, again all browsers are latest versions with no extensions):

Here's a screenshot of a file in Safari:

And here's the same file (with an extra line of text) in Firefox 38.0.5:

Notice how, in Safari, the font is not displayed well (wrong kerning, spacing, scaling, etc.). This problem occurs for *all* fonts, not just Helvetica.