diff --git a/src/calibre/ebooks/oeb/stylizer.py b/src/calibre/ebooks/oeb/stylizer.py
index fdb5e44195..4f662ecdf8 100644
--- a/src/calibre/ebooks/oeb/stylizer.py
+++ b/src/calibre/ebooks/oeb/stylizer.py
@@ -428,10 +428,23 @@ class Stylizer:
 
 
 no_important_properties = frozenset()
+svg_text_tags = tuple(map(SVG, ('text', 'textPath', 'tref', 'tspan')))
+
+
+def is_only_number(x: str) -> bool:
+    try:
+        float(x)
+        return True
+    except Exception:
+        return False
+
+def is_svg_text_tag(x):
+    return getattr(x, 'tag', '') in svg_text_tags
 
 
 class Style:
     MS_PAT = re.compile(r'^\s*(mso-|panose-|text-underline|tab-interval)')
+    viewport_relative_font_size: str = ''
 
     def __init__(self, element, stylizer):
         self._element = element
@@ -628,6 +641,8 @@ class Style:
                 base = self._profile.fbase
             if 'font-size' in self._style:
                 size = self._style['font-size']
+                if is_svg_text_tag(self._element) and (size.endswith('px') or is_only_number(size)):
+                    self.viewport_relative_font_size = size
                 result = normalize_fontsize(size, base)
             else:
                 result = base
diff --git a/src/calibre/ebooks/oeb/transforms/flatcss.py b/src/calibre/ebooks/oeb/transforms/flatcss.py
index fae357e8d3..76ca6fd9e6 100644
--- a/src/calibre/ebooks/oeb/transforms/flatcss.py
+++ b/src/calibre/ebooks/oeb/transforms/flatcss.py
@@ -453,7 +453,9 @@ class CSSFlattener:
             if dp.tag and dp.tag.endswith('}div') and len(dp) == 1 and not dp.text:
                 if stylizer.style(dp).cssdict().get('float', None) == 'left':
                     is_drop_cap = True
-        if not self.context.disable_font_rescaling and not is_drop_cap:
+        if style.viewport_relative_font_size:
+            cssdict['font-size'] = style.viewport_relative_font_size
+        elif not self.context.disable_font_rescaling and not is_drop_cap:
             _sbase = self.sbase if self.sbase is not None else \
                 self.context.source.fbase
             dyn_rescale = node.attrib.pop('data-calibre-rescale', None)