How to Over-Ride CSS with User-CSS on Mobile Safari

CSS style override doesn't work on mobile browsers

please remove bgcolor="#000000" from your body element and everything will work fine.

Override a specific css class only for Safari browser

-webkit-appearance is (luckily or not) also supported by Firefox and Edge as mentioned in the mozilla documentation.
Only thing I can imagine to work is to use JavaScript to target the user agent:

function detectBrowser() {
  if (navigator.userAgent.includes("Safari")) {
    // your code here
  }
}

Why does the iOS Safari user agent stylesheet appear to override my margin style on the body?

This appears to be a bug in Safari on iPad with iOS7. It remains in 7.0.6.

iOS 7 iPad Safari Landscape innerHeight/outerHeight layout issue

iphone overriding all of my styles on an input[type=submit]

You might be looking for:

-webkit-appearance: none;

CSS Webkit Appearance Override?

css-infos.net lists the values as:

caps-lock-indicator The indicator that appears in a password field when Caps Lock is active. Available in Safari 4.0 and later. Available in iOS 2.0 and later.
button  
button-bevel    
caret   
checkbox    
default-button  
listbox 
listitem    
media-fullscreen-button 
media-mute-button   
media-play-button   
media-seek-back-button  
media-seek-forward-button   
media-slider    
media-sliderthumb   
menulist    
menulist-button 
menulist-text   
menulist-textfield  
none    
push-button 
radio   
searchfield 
searchfield-cancel-button   
searchfield-decoration  
searchfield-results-button  
searchfield-results-decoration  
slider-horizontal, slider-vertical  
sliderthumb-horizontal  
sliderthumb-vertical    
square-button   
textarea    
textfield   
scrollbarbutton-down    Unsupported in Safari 4.0
scrollbarbutton-left    Unsupported in Safari 4.0
scrollbarbutton-right   Unsupported in Safari 4.0
scrollbarbutton-up  Unsupported in Safari 4.0
scrollbargripper-horizontal Unsupported in Safari 4.0
scrollbargripper-vertical   Unsupported in Safari 4.0
scrollbarthumb-horizontal   Unsupported in Safari 4.0
scrollbarthumb-vertical Unsupported in Safari 4.0
scrollbartrack-horizontal   Unsupported in Safari 4.0
scrollbartrack-vertical Unsupported in Safari 4.0