Oscilloscope.css: slow scrolling on Chrome, Safari

[chrstphrchvz]

I encountered this issue while reading the HSW12ASM online manual recently. It occurs in Chrome and Safari but not in Firefox or Microsoft browsers. Disabling text-shadow seems to workaround it, so I think it is a performance issue with WebKit or the browsers rather than the CSS code, although I haven't looked for reports of an upstream issue. Disabling text-shadow also doesn't seem to affect the readability, so that may be an option.