In the Composer's Words 
"I take the liberty of giving a brief summary of my intentions in the performance of the music of this opera ..."

Scarlatti enjoyed the patronage of Ferdinando de' Medici in the first decade of the 18th century. The cultured prince - who supported the ambitious inventions of Bartolomeo Cristofori while he developed the 'cembalo col forte e piano' - commissioned several operas from Alessandro. The composer could not often attend to direct the productions, and he entrusted some of his ideas about his music to the prince. 

Light and shade

'Ho notato ... a' luoghi opportuni, i piani e forti degl'Istromenti che sono unicamente il chiaroscuro che fanno aggradevole qualsivoglia canto e suono.
(Alessandro Scarlatti to Ferdinando de' Medici - 1706.)

'I have marked ... at necessary places the louds and softs of the instruments, which are the only means of making that chiaroscuro which makes singing and playing attractive.'
(A. Scarlatti, on his opera, Il Gran Tamerlano 1706.)

Is this one of the earliest examples of the concept chiaroscuro being applied to dynamic contrasts in music?

The composer's concept of his own style

The singer experienced in Scarlatti's style was entrusted with instructing the singers in Florence, when Scarlatti would not be present at the production of his opera Turno Aricino. Alessandro gave detailed instructions to the castrato Sigr. Matteucci - 
'come professore prattico del mio stile, a cui ho pregato di comunicarlo a' Signori Virtuosi attori.' (Rome, 1704)

Tempi 

'un breve ristretto della mia intenzione nel portamento della Musica di quest' Opera
(Lucio Manlio, Florence 1705)

 ''dove segnato grave, non intendo malenconico;
dove andante, non presto ma arioso;
dove allegro, non precipitoso;
dove allegrissimo, tale che non affanni il Cantante n affoghi le parole;
dove andante-lento, in forma che escluda il patetico, ma sia un amoroso vago che non perda l'arioso.
E in tutte l'arie, nessun malenconica."

(Alessandro Scarlatti to Ferdinando de' Medici, Rome 1705)

"Where it is marked grave, I don't mean melancholy;
where andante, not quickly, but pleasing.
where allegro, not rushing;
where allegrissimo, so that it doesn't trouble the singer nor choke the words;
where andante-lento, in a way that exludes the pathetic, but let it have an
amorous beauty that doesn't lose tunefulness;
And in all the arias, nothing melancholy"

(Did Alessandro feel under attack about being termed a composer of melancholy music?)

All extracts from letters in the Archivio di Stato, Firenze, and quoted in Mario Fabbri, Alessando Scarlatti e il Principe Ferdinando de' Medici, Firenze, Leo S. Olschki, 1961