





Похоже, Safari не нравится тег <base>. Это останавливает работу заливки в этом браузере, поэтому вы получаете заливку по умолчанию, которая является черной.
например
html, body {
width: 100%;
height: 100%;
}<base href = "http://emtre.ch.tajo.host.ch/" />
<svg xmlns = "http://www.w3.org/2000/svg" xmlns:xlink = "http://www.w3.org/1999/xlink" id = "footer-svg" width = "100%" height = "100%" viewBox = "0 0 2000, 1200">
<defs>
<linearGradient id = "grad1" x1 = "0%" y1 = "0%" x2 = "100%" y2 = "0%">
<stop offset = "0%" style = "stop-color:#efefef;stop-opacity:1"></stop>
<stop offset = "100%" style = "stop-color:#FFF;stop-opacity:1"></stop>
</linearGradient>
</defs>
<polygon points = "0,595 1903,0 1903,1010 0,1010" fill = "url(#grad1)"></polygon>
</svg>
против
html, body {
width: 100%;
height: 100%;
}<svg xmlns = "http://www.w3.org/2000/svg" xmlns:xlink = "http://www.w3.org/1999/xlink" id = "footer-svg" width = "100%" height = "100%" viewBox = "0 0 2000, 1200">
<defs>
<linearGradient id = "grad1" x1 = "0%" y1 = "0%" x2 = "100%" y2 = "0%">
<stop offset = "0%" style = "stop-color:#efefef;stop-opacity:1"></stop>
<stop offset = "100%" style = "stop-color:#FFF;stop-opacity:1"></stop>
</linearGradient>
</defs>
<polygon points = "0,595 1903,0 1903,1010 0,1010" fill = "url(#grad1)"></polygon>
</svg>