IE9 Document Mode Toggle fixes SharePoint CSS issues?

I'm working on my first SharePoint project doing some UX support. My client's page uses some custom css including a horizontal main-menu with dropdown sub-menus. When the page is loaded in IE9, none of the sub-menus work correctly. The dropdowns appear when you hover over the top-level item but when you drop your cursor down to the sub-menu, the hover doesn't register and the menu disappears.

These links work fine in latest versions of Chrome and Firefox, so I'm pretty sure this is an IE quirk. In IE9, if I open the developer tools and toggle the document mode from IE9 to IE7 and then back to IE9, the menus work correctly for the remainder of the browser tab's lifetime (e.g. when I use the sub-menus to navigate to other pages, the dropdowns continue to function properly). Unfortunately, since this is SharePoint, I don't have a lot of control over the structure of the html开发者_如何学编程 here.

Anyone know what might be going on? Does anyone know the impact of toggling the browser modes to IE7 and back and why that might fix the problem? Thanks!

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Try this:

<meta http-equiv="X-UA-Compatible" content="IE=7">

It will force IE8, IE9 etc to render as if they were IE7.