splFormSearch is included only on the <form> style Simple XML views. However when you use the ?showsource=1 trick to convert from Simple XML to advanced XML, you'll inadvertently bring this little objectMode="SimpleForm" attribute into your Advanced XML, and its presence there is enough to turn on the class="splFormSearch" behavior in the rendered view. So anyway, in effect the attribute can commonly be found in advanced XML too.
I wouldn't advise relying on this. For one thing I wouldn't trust Splunk not to change all this CSS when there's a major version change. For another this is only going to style all the core splunk elements, and since you've tagged the question with 'sideview' I assume you're using TextField instead of ExtendedFieldSearch, and Pulldown instead of StaticSelect and SearchSelectLister and so on and so forth.
By convention I put all form element modules up in the "mainSearchControls" layoutPanel, and although they all do have "float" params ( <param name="float">left</param> ), since that style doesn't get applied for a second or two after the page loads, there's a jitter effect. So I lock it down by putting custom CSS into the page as well. Here's a simple starting point.
.mainSearchControls .TimeRangePicker,
.mainSearchControls .TextField,
.mainSearchControls .Pulldown,
.mainSearchControls .Checkbox,
.mainSearchControls .Checkboxes {
float:left;
}
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