CSS validation error

Hi guys! I have been working my way through the Wordpress 2014 theme CSS, (style.css) file which I plan to use for my site. I get a few of errors throughout my css when I validate. Here is the code I’m working with.

[ .gallery-columns-2 .gallery-item {
max-width: 48%;
max-width: -webkit-calc(50% - 4px);
max-width: calc(50% - 4px);

    .site-title {
float: left;
font-size: 18px;
font-weight: 700;
line-height: 48px;
margin: 0;
 /* Nav-toggle width + search-toggle width - gutter = 86px */        
 max-width: -webkit-calc(100% - 86px);
 max-width:         calc(100% - 86px);

All the error messages I’m getting have to do with the same issue so I’m hoping if I fix one, its the same style of fix for them all. This is what I receive:

The “max-width” property has an invalid value “calc(100% - 86px)”. The “min-width”, “min-height”, “max-width”, and “max-height” properties allow authors to constrain content widths and heights to a certain range. This property must have one of the following values: “none”, “inherit”, a valid length, or a valid percent.

I am assuming this CSS has to do with responsive design? Any help is appreciated! thanks!

I think maybe the validator just got out of the wrong side of the bed today. Perhaps it hasn’t been told about calc() yet. It’s quite valid, but also new. I wouldn’t worry. It works fine, so it’s not an issue.