apankrat/js-noisy-gradient – js библиотека для сглаживания линейного градиента

var nvg = new NoisyVerticalGradient(50, 400, ['#112233', '#223344'] );
var png = nvg.render_png();
document.getElementById('xyz').style.backgroundImage = png;

https://github.com/apankrat/js-noisy-gradient