Both meter elements should have a nested shadow box with a width specified:
| "
    "
| <meter>
|   max="100"
|   value="70"
|   <shadow:root>
|     <div>
|       pseudo="-webkit-meter-inner-element"
|       shadow:pseudoId="-webkit-meter-inner-element"
|       <div>
|         pseudo="-webkit-meter-bar"
|         shadow:pseudoId="-webkit-meter-bar"
|         <div>
|           pseudo="-webkit-meter-optimum-value"
|           style="inline-size: 70%; block-size: 100%;"
|           shadow:pseudoId="-webkit-meter-optimum-value"
|     <div>
|       pseudo="-internal-fallback"
|       shadow:pseudoId="-internal-fallback"
|       <slot>
| "
    "
| <meter>
|   high="6"
|   low="3"
|   max="10"
|   min="0"
|   optimum="5"
|   value="10"
|   <shadow:root>
|     <div>
|       pseudo="-webkit-meter-inner-element"
|       shadow:pseudoId="-webkit-meter-inner-element"
|       <div>
|         pseudo="-webkit-meter-bar"
|         shadow:pseudoId="-webkit-meter-bar"
|         <div>
|           pseudo="-webkit-meter-suboptimum-value"
|           style="inline-size: 100%; block-size: 100%;"
|           shadow:pseudoId="-webkit-meter-suboptimum-value"
|     <div>
|       pseudo="-internal-fallback"
|       shadow:pseudoId="-internal-fallback"
|       <slot>
| "
    "
| <meter>
|   high="6"
|   low="3"
|   max="10"
|   min="0"
|   optimum="0"
|   value="10"
|   <shadow:root>
|     <div>
|       pseudo="-webkit-meter-inner-element"
|       shadow:pseudoId="-webkit-meter-inner-element"
|       <div>
|         pseudo="-webkit-meter-bar"
|         shadow:pseudoId="-webkit-meter-bar"
|         <div>
|           pseudo="-webkit-meter-even-less-good-value"
|           style="inline-size: 100%; block-size: 100%;"
|           shadow:pseudoId="-webkit-meter-even-less-good-value"
|     <div>
|       pseudo="-internal-fallback"
|       shadow:pseudoId="-internal-fallback"
|       <slot>
| "
  "