Validation CSS 3.0 not a known property name

Question

I have multiple CSS pages and every CSS property gets the following warning:

Validation(CSS 3.0): "[]" is not a known property name.

I can run my code, that's not the problem as it's only a warning.

It wouldn't bother me if my code asked what properties are possible to use? I've searched for 2 days now and also reinstalled my VS2015. My colleague doesn't have that problem on the same program.

Any suggestions about what could be wrong?

A bit Code (everything is green underlined):

html, body {
    padding: 0;
    margin: 0;
    background-color: #CCC;
    overflow: hidden;
}

.wrapper {
    position: fixed;
    background-color: #CCC;
    -moz-box-sizing: border-box;
    -webkit-box-sizing: border-box;
    box-sizing: border-box;
    width: 100%;
    height: 100%;
    padding: 0;
    left: 0px !important;
    top:  0px !important;
    margin-bottom: 0px;
}

.menu {
    position: fixed;
    background-color: #333;
    -moz-box-sizing: border-box;
    -webkit-box-sizing: border-box;
    box-sizing: border-box;
    width: 320px;
    height:100%;
    padding: 0;
    left: 0px !important;
    top: 0px !important;
    display: flex;
    -ms-flex-direction: column;
    flex-direction: column;
    align-items: flex-start;
    -ms-flex-wrap: wrap;
    flex-wrap: wrap;

}

Hope thats clear enough ;)

Solution 1

Adding Web Essentials solved the problem.

Solution 2

In case people are still searching for an answer to this puzzling issue, I found restarting visual studio resolved it for me.

Solution 3

In visula studio CSS version would have selected to CSS3.0 change it to older the problem will be solved.