Polymer 1.0 on Firefox, Safari /deep/ selector not working

Question

Can someone please explain to me why in Safari and Firefox, things like

Polymer.dom(document.body).querySelectorAll('body /deep/ div.some-class')

result in SyntaxError: An invalid or illegal string was specified?

I thought deep linking was polyfilled on non-shadow-DOM browsers.

I also get the same error thrown on selectors with /deep/ in them, if they define a CSS "variable" upon document load

For example, my main index.php contains <link rel="import" href="shell/styles.html"/>, which has the following:

html /deep/ paper-button { /* OK */
    height: 46px;
}

html /deep/ paper-icon-button { /* Throws error */
    --paper-icon-button-ink-color: var(--colorWhite);
}

Any insight will be appreciated.