// Callback used by the HTML window we create below
function attributeWindowCallback(val) 
{
    let obj = JSON.parse(val);
    if (obj.StrokesPlusHTMLWindow) {
        var handle = new IntPtr(parseInt(obj.StrokesPlusHTMLWindow.Handle))
        sp.StoreHandle("attributeWindowHandle",handle);
    } 
    else if (obj.action)
    {
        switch(obj.action) {
            case "Close":
                sp.WindowFromHandle(sp.GetStoredHandle("attributeWindowHandle")).SendClose();        
                break;
        }
    }
}

// Opens an HTML window to a URL (this post), waits 2 seconds, then looks for a div and shows an 
// alert with the html of the element, then closes the HTML window
sp.HTMLWindow("Find Element", //Window title
              "https://forum.strokesplus.net/posts/t8539-Can-the-Sprogram-obtain-the-attribute-values-of-the-specified-elements-in", 
              "attributeWindowCallback", //message callback
              `setTimeout(function(){ // Script to execute on document create
                 alert($('.selectionQuoteable').html());
                 window.chrome.webview.postMessage({ action: 'Close'});
               }, 2000);`, // Waits 2 seconds, then gets an element via jQuery
              "", // window ID
              false); // include jQuery/bootstrap - ignored for file/url