You should use media queries to target specific devices. The general schema for a media query is like below:
@media not|only mediatype and (expressions) {
/* your css rules */
}
You can exclude screens smaller than a certain width from your CSS rules by setting them to their defaults,
p, img{
display: flex;
}
img{
float: left;
width: 45%;
padding: 0;
margin: 0;
}
p{
float: right;
text-align: justify;
width: 50%;
padding: 0;
margin: 0;
font-size: 20px;
}
@media only screen and (max-width: 768px) {
p {
display: block;
}
img {
display: inline-block;
}
}
or you can write your rules such that they're only applied to screens larger than a certain width:
@media only screen and (min-width: 768px) {
p, img {
display: flex;
}
}
img{
float: left;
width: 45%;
padding: 0;
margin: 0;
}
p{
float: right;
text-align: justify;
width: 50%;
padding: 0;
margin: 0;
font-size: 20px;
}
This reference should help for media queries:
https://www.w3schools.com/css/css3_mediaqueries.asp
There is also a good resource on various ways to set a display property to default values which can be found here:
Reset CSS display property to default value