While this can make rule-creation difficult, it shouldn’t affect the behavior/performance of your custom rules in actual API calls. However to correctly construct rules you may need to use your regular browser’s “Inspect” functionality to identify the proper CSS selectors to use.
Sometimes our rendering engine is redirected to a mobile or alternate-device template. This too can cause a visual difference between what you see in your regular browser and what you see in the editor preview. Our preview window ususally matches what our API sees, so please construct your rules based on the editor preview. For a step by step tutorial on how to do this, please read this.