Hi Macchi,
As I understand it, you're trying to specify the font size based on the viewport width - which is a pretty cool idea, but it might be easier to control the font size using media queries instead, using relative length units such as "em" or "vw", and removing the movie description from the poster for mobile view.
I also think the class names you've created in your HTML file are too generic and might be affecting styles in your Squarespace site which is where some problems may be arising.
I've attached an example I made in Squarespace using an image block with the poster