Defining CSS media queries within selectors

Short answer, no. There are no performance issues in defining media queries within CSS selectors.

But let's dive in...

As described in Anselm Hannemann great article Web Performance: One or Thousands of Media Queries there is no performance loss from adding the media queries in the manner you are.

As long as the same set of media queries are being used in each selector there is no major performance hit other than your CSS file might be a bit larger.

.foo {
  @media (min-width: 600px) { ... }
  @media (min-width: 1000px) { ... }
  @media (min-width: 1500px) { ... }
}

.bar {
  @media (min-width: 600px) { ... }
  @media (min-width: 1000px) { ... }
  @media (min-width: 1500px) { ... }
}

However, it does matter how many different media queries you use. Different being different min-widths, max-widths and so on.

There shouldn't be a performance difference looking at the way browsers handle media queries. Browser engines do serialize and strip out duplicated media-queries so they only need to evaluate each media query once. Also they cache the queries so that they can re-use it later on. It doesn’t matter if you use one big or multiple media-queries in your code assuming your values are mostly the same.

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Some of the possibilities when there can be performance issues

• You use multiple media queries with different values and the browser window is re-sized. As the browser window is re-sized, multiple media queries can be a big overhead on the cpu.
• When the CSS selectors are too complex. A complex CSS selector is much performance hindering than multiple media queries. So having multiple media queries inside complex selectors can cause performance issues