Even though vendor-specific extensions are guaranteed not to cause conflicts (unless two vendors happen to choose the same identifier, of course), it should be recognized that these extensions may also be subject to change at the vendor’s whim, as they don’t form part of the CSS specifications, even though they often mimic the proposed behavior of existing or forthcoming CSS properties.
Although these extensions can be useful at times, it’s still recommended that you avoid using them unless it’s absolutely necessary. It’s also worth noting that, as is usually the case with proprietary code, the extensions will not pass CSS validation.
Having said all of the above you have some errors in your linear gradient code. Vendor specific code comes before standard code. You're also missing the vendor specific code for IE plus you have one too many sets of rgba code in your code. I rewrote it below:
background-image: -webkit-linear-gradient(left, rgba(0,0,0,0), rgba(0,0,0,0.75)); /* vendor specific code for webkit browsers */
background-image: -moz-linear-gradient(left, rgba(0,0,0,0), rgba(0,0,0,0.75)); /* mozilla */
background-image: -o-linear-gradient(left, rgba(0,0,0,0), rgba(0,0,0,0.75)); /* opera */
background-image: -ms-linear-gradient(left, rgba(0,0,0,0), rgba(0,0,0,0.75)); /* IE10 */
filter: progid:DXImageTransform.Microsoft.gradient(startColorstr=#99000000, endColorstr=#9900000 /* IE 5.5-7 */
-ms-filter: progid:DXImageTransform.Microsoft.gradient(startColorstr=#99000000, endColorstr=#99000000 /*IE8 */
background-image: linear-gradient(left, rgba(0,0,0,0), rgba(0,0,0,0.75)); /* standard code */