diff --git a/R/utils.R b/R/utils.R
index 4f2c21f..ff4ddc1 100644
--- a/R/utils.R
+++ b/R/utils.R
@@ -75,6 +75,46 @@
   sprintf("data:%s;base64,%s", mime, base64enc::base64encode(path))
 }
 
+# qrcode::generate_svg() paints the code -- quiet zone included -- into a
+# fixed 300x300 canvas and emits no viewBox, so the svg cannot be resized by
+# CSS: given a smaller box it keeps drawing at full size and only its top
+# left corner remains visible. Give it a viewBox cropped to the dark modules
+# instead. The code then scales to whatever box it is placed in, and its
+# quiet zone becomes the surrounding layout's business (on the title slide
+# the white plate behind the code supplies it), which keeps the placement
+# independent of how many modules the encoded url happens to need.
+qr_svg_fit <- function(svg) {
+  rects <- regmatches(svg, gregexpr("<rect[^>]*fill=\"black\"[^>]*>", svg))[[1]]
+  attr_num <- function(name) {
+    suppressWarnings(as.numeric(
+      sub(sprintf(".*[ ]%s=\"([0-9.]+)(px)?\".*", name), "\\1", rects)
+    ))
+  }
+  x <- attr_num("x")
+  y <- attr_num("y")
+  w <- attr_num("width")
+  h <- attr_num("height")
+  if (!length(rects) || anyNA(c(x, y, w, h))) {
+    return(svg)
+  }
+  left <- min(x)
+  top <- min(y)
+  side <- max(max(x + w) - left, max(y + h) - top)
+  vb <- sprintf(
+    'viewBox="%s %s %s %s" width="%s" height="%s"',
+    format(left), format(top), format(side), format(side),
+    format(side), format(side)
+  )
+  root <- regmatches(svg, regexpr("<svg[^>]*>", svg))
+  fitted <- sub(
+    "<svg", paste("<svg", vb),
+    gsub("[ ](width|height)=\"[0-9.]+\"", "", root),
+    fixed = TRUE
+  )
+  # the xml prolog would end up as a stray comment once inlined into html
+  sub("^<[?]xml[^>]*[?]>", "", sub(root, fitted, svg, fixed = TRUE))
+}
+
 #' HTML code for a linked QR code
 #'
 #' Generates a QR code (inline SVG) linking to the given URL and returns the
@@ -111,7 +151,7 @@
     ""
   }
 
-  svg <- gsub("\r?\n|\r", "", readChar(tmp, 1e7))
+  svg <- qr_svg_fit(gsub("\r?\n|\r", "", readChar(tmp, 1e7)))
   qr <- sprintf('<a class="qrcode" href="%s">%s</a>', url, svg)
   if (nchar(cap_html) > 0) {
     paste0('<div class="qrcode-container">', qr, cap_html, "</div>")