I am adding responsive feature to a website using css, this website has been their for sometime and now responsive feature is being added to it so that it can support mobile ver
This isn't really an issue you can solve. Your code is fine.
Below you'll see an image of a few screenshots I took. I ran the Fiddle through Android 2.3.3 and 4.4 on virtual machines, then played with the scaling. The top two pictures are 2.3.3 and 4.4, scaled to 100% resolution. 1 pixel on my screen is 1 pixel on the 'phone'. You can see that the lines are fine (I think the 4.4 screenshot might not have been exactly 1:1, but its clear in the 2.3.3 screenshot)
But below, you will see where I have taken the same device but scaled the phone down, so it was no longer 1:1. You can see the issue you are describing appearing in these screenshots, meaning that the way your phone screen and the scaling is the issue here.
Edit: You might need to load the image URL in a new window to see the real thing. I'm not sure how to link a picture on here sorry!