The Alert method shows a popup message on the screen which stays on screen until the user presses the Ok button or back button.

 app.Alert( text, title, options, hue );

function OnStart()
{
  app.Alert( "Hello World!" );
}

function OnStart()
{
  app.Alert( "Hello World!", "Message" );
}

The argument hue represents the color of the title text and the horizontal line below.

//Orange title and horizontal line
function OnStart()
{
  app.Alert( "Hello World!", "Message", "", 180 );
}

Normally it is not possible to use HTML tags in an alert. But if you add the following function to your script

function HtmlAlert( text, clr1, title, clr2, bclr )
{
    //background-layout
    htmlAlertLayout = app.CreateLayout( "linear", "VCenter,FillXY" );
    htmlAlertLayout.Animate( "FadeIn", "", 100 );
    htmlAlertLayout.SetBackColor( [bclr.slice(0, 1), "33", bclr.slice(1)].join('') );
 
    //alert-layout
    htmlAlert = app.CreateLayout( "linear", "Left" );
    htmlAlert.SetBackColor( bclr );
    htmlAlert.SetSize( 0.8, 0.2 );
 
    //title-text
    htmlAlertTitle = app.CreateText( title );
    htmlAlertTitle.SetTextSize( 30 );
    htmlAlertTitle.SetMargins( 0.02 );
    htmlAlertTitle.SetTextColor( clr2 );
 
    //horizontal line below
    htmlAlertHr = app.CreateText( "", 1, 0.002 );
    htmlAlertHr.SetBackColor( clr2 );
 
    //text in alert
    htmlAlertText = app.CreateText( "", 0.8, 100, "Multiline,Left");
    htmlAlertText.SetMargins( 0.02 );
    htmlAlertText.SetHtml( text );
    htmlAlertText.SetTextColor( clr1 );
 
    //horizontal line below
    htmlAlertHr2 = app.CreateText( "", 0.8, 0.002 );
    htmlAlertHr2.SetBackColor( clr2 );
 
    //"Ok" button
    htmlAlertButton = app.CreateText( "Ok", 0.8, 0.055 );
    htmlAlertButton.SetPadding( 0, 0.01 );
    htmlAlertButton.SetBackColor( bclr );
    htmlAlertButton.SetTextColor( clr1 );
    htmlAlertButton.SetOnTouchUp( function() { htmlAlertLayout.Animate( "FadeOut", "", 100 ); app.RemoveLayout( htmlAlertLayout ); } );
 
    //add objects to app
    htmlAlert.AddChild( htmlAlertTitle );
    htmlAlert.AddChild( htmlAlertHr );
    htmlAlert.AddChild( htmlAlertText );
    htmlAlertLayout.AddChild( htmlAlert );
    htmlAlertLayout.AddChild( htmlAlertHr2 );
    htmlAlertLayout.AddChild( htmlAlertButton );
    app.AddLayout( htmlAlertLayout);
}

and call the method for example like that

//Called when application is started.
function OnStart()
{
    app.EnableBackKey( false );
    lay = app.CreateLayout( "linear" );
    lay.SetSize( 1, 1 );
    lay.SetBackColor( "#fff56f" );
    app.AddLayout( lay );
}
 
function OnBack()
{
    HtmlAlert( "<s>text</s><br><b>text</b><br><u>text</u>", "#fff56f", "title", "#fff56f", "#121212" );    
}

Then you are able to use HTML tags in an alert.