When you are writing media queries, it makes it super difficult to debug and find out what's happening when your queries are sprinkled throughout. When it comes to media queries, grouping by feature (header, main content, etc) is not the best solution. Instead, group by breakpoints. You start with the general code that applies all the time, then you start the breakpoints from largest to smallest. I would recommend going through the Google Responsive Design guidelines. They provide examples and explanations of how and when to use breakpoints.
When you are writing media queries, it makes it super difficult to debug and find out what's happening when your queries are sprinkled throughout. When it comes to media queries, grouping by feature (header, main content, etc) is not the best solution. Instead, group by breakpoints. You start with the general code that applies all the time, then you start the breakpoints from largest to smallest. I would recommend going through the Google Responsive Design guidelines. They provide examples and explanations of how and when to use breakpoints.