rx

The rx attribute defines a radius on the x-axis.

Two elements are using this attribute: <ellipse>, and <rect>

<svg viewBox="0 0 300 200" xmlns="http://www.w3.org/2000/svg">
  <ellipse cx="50"  cy="50" rx="0"  ry="25" />
  <ellipse cx="150" cy="50" rx="25" ry="25" />
  <ellipse cx="250" cy="50" rx="50" ry="25" />

  <rect x="20"  y="120" width="60" height="60" rx="0"   ry="15"/>
  <rect x="120" y="120" width="60" height="60" rx="15"  ry="15"/>
  <rect x="220" y="120" width="60" height="60" rx="150" ry="15"/>
</svg>

ellipse

For <ellipse>, rx defines the x-radius of the shape. With a value lower or equal to zero the ellipse won't be drawn at all.

Note: Starting with SVG2, rx is a Geometry Property meaning this attribute can also be used as a CSS property for ellipses.

rect

For <rect>, rx defines the x-axis radius of the ellipse used to round off the corners of the rectangle.

The way the value of the rx attribute is interpreted depend on both the ry attribute and the width of the rectangle:

• If a properly specified value is provided for rx but not for ry (or the opposite), then the browser will consider the missing value equal to the defined one.
• If neither rx nor ry has a properly specified value, then the browser will draw a rectangle with square corners.
• If rx is greater than half of the width of the rectangle, then the browser will consider the value for rx as half of the width of the rectangle.

Note: Starting with SVG2, rx is a Geometry Property meaning this attribute can also be used as a CSS property for rects.

Specifications