Adding a title element, or an alt attribute to image/svg+xml;base64 in the CSS

title = Global attribute
Text to be displayed in a tooltip when hovering over the element.

SVG <title>

Each container element or graphics element in an SVG drawing can supply a <title> element containing a description string where the description is text-only. When the current SVG document fragment is rendered as SVG on visual media, <title> element is not rendered as part of the graphics. However, some user agents may, for example, display the <title> element as a tooltip. Alternate presentations are possible, both visual and aural, which display the <title> element but do not display path elements or other graphics elements. The <title> element generally improves accessibility of SVG documents.

Generally <title> element should be the first child element of its parent. Note that those implementations that do use <title> to display a tooltip often will only do so if the <title> is indeed the first child element of its parent.