If you are playing the single-player offline version (the HTML file saved to your computer), you can use the "Extract and Refresh" trick.
Because Eaglercraft stores the world data in your browser's IndexedDB, you cannot simply drag files in. However, recent versions of Eaglercraft (Offline Download v1.8.8) allow you to import a resource pack via the "Settings" menu using file upload. eaglercraft 188 resource packs work
Resource packs in Eaglercraft 1.8.8 work remarkably well for a web-based port, but they do not have 100% compatibility with standard Java Edition files. If you understand the limitations, you can drastically change the look of the game without crashing your browser. If you are playing the single-player offline version
This is the primary way to ensure Eaglercraft 1.8.8 resource packs work on any server or single-player world. You need a direct download link to a .zip file hosted online. This is the primary way to ensure Eaglercraft 1
Step-by-step guide:
If the URL is valid and the pack is for 1.8.8, the textures will apply immediately.