Q:

How to fix Avoid AlphaImageLoader filter?

I try to increase speed of my website and I use gtmetrix for fix speed problems.
I get a message that 

Avoid AlphaImageLoader filter
There is a total of 2 filters
https://www.travelenter.com/bundles/bootstrapCss?v=EXugHAHZ_FGhi2mjw1qnjtce7yGY8egZz7H9mEwgBxM1

I try to fix this problem and found that if we use filter: alpha(opacity=0); for set opacity of png image , this Makes that load of web page stop until load this image and then continue .I delete all css code such as filter: alpha(opacity=0); of my css files and retest but don't solved .
How to fix this?
gtmetrix link: 
https://gtmetrix.com/reports/www.travelenter.com/O0zZGkzL

A:

I used number code such as
-ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=50)"; in my css file for png image that after delete this codes my problem solved.

