Use UIWebView's `loadHTMLString:baseURL:` and set the baseURL to where your assets are in the app bundle. Then replace all absolute paths in your html/js/css with relative ones using something like the script below. The relative URLs will be loaded relative to the baseURL, whereas absolute ones are loaded relative to the root of the filesystem, …

fixPrefixes.m

// This doodad converts absolute paths like "/assets/thing.jpg" into relative ones like "assets/thing.jpg"
NSString * (^fixPrefix)(NSString *, NSString *) = ^NSString *(NSString * input, NSString *prefix) {
    NSString *needle = [[@"/" stringByAppendingString:prefix] stringByAppendingString:@"/"];
    NSString *replacement = [prefix stringByAppendingString:@"/"];
    
    input = [input stringByReplacingOccurrencesOfString:[@"'" stringByAppendingString:needle] withString:[@"'" stringByAppendingString:replacement]];
    input = [input stringByReplacingOccurrencesOfString:[@"\"" stringByAppendingString:needle] withString:[@"\"" stringByAppendingString:replacement]];
    input = [input stringByReplacingOccurrencesOfString:[@"(" stringByAppendingString:needle] withString:[@"(" stringByAppendingString:replacement]];
    return input;
};

fileContents = fixPrefix(fileContents, @"assets";