Formatting not obeyed in some internet browsers

I've been building a site using Chrome and just looked at it using Safari. Each page has an image background with a semi-transparent overlay for readability purposes; Safari seems to be completely ignoring and neglecting the semi-transparent overlay on certain pages. This makes those pages essentially illegible when viewed on Safari or any other browser that similarly ignores my formatting.

Does anyone know why this is happening—or more importantly, how I can fix this and avoid a similar issue in the future? Thanks