Would these dropcaps work for Kindle formats?

Am editing my book as an epub in Sigil and plan to create the Kindle format using Kindlegen.

I found this CSS for creating dropcaps. It seems to work on my Kindle3 device - how safe is this to use?

There are two styles, I'm guessing to allow for different letter shapes.

.dropcap {
float: left;
font-size: 4.9437rem;
font-style: normal;
font-variant: normal;
font-weight: normal;
margin: -0.2em 0.1em -0.25em 0
}
.dropcap1 {
float: left;
font-size: 1.5em;
font-style: normal;
font-variant: normal;
font-weight: normal;
line-height: 1.2;
margin: -0.2em 0.1em -0.25em 0
}