Center alignment without Flexbox

Apologies if this question is too basic but it’s something I really struggle with. Native sliders won’t allow me to use flexbox for individual slider and I see no options to centre its content.

@mkmiki
Lets say I have a div and inside is a child div. If I want to center the child div then I need to give it width and set the margin-left and margin-right to auto. That centers an element without flex.

When working with slides a common approach is to add a div to the slide element (inside the Mask) as a content container and give it a height of 100%. You can then set that elements display to flex to align children accordingly.