Yes, you can load a html string by calling
LoadHTMLString method on the webview component. You can specify a base url as a search path for your resources referred in your html string. Please see the script reference and demo code in the package for more.
Notice: you need to encode your base url correctly when you use it. It should be a valid path in the target system. For example, if you put an image file with name "my_image.png" under the Streaming Assets folder and try to load it in a plain html string like this:
In iOS, the baseUrl should be:
"file:/" + Application.streamingAssetsPath.Replace("/","//").Replace(" ","%20") + "//"
while in Android, it should be:
You could also refer to this post for more information on it.