async getArtifact(context) {
    const session = context.driver.defaultSession;
    const executionContext = context.driver.executionContext;

    await session.sendCommand('DOM.enable');
    await session.sendCommand('CSS.enable');
    await session.sendCommand('CSS.startRuleUsageTracking');

    // Force style to recompute.
    // Doesn't appear to be necessary in newer versions of Chrome.
    await executionContext.evaluate(() => window.getComputedStyle(document.body), {
      args: [],
    });

    const {ruleUsage} = await session.sendCommand('CSS.stopRuleUsageTracking');
    await session.sendCommand('CSS.disable');
    await session.sendCommand('DOM.disable');

    return ruleUsage;
  }
@param {LH.Gatherer.Context} context
@return {Promise<LH.Artifacts['CSSUsage']>}