Welcome to our forum & thanks for the question!
As you can see in the API (https://api.highcharts.com/highcharts/chart.width
is a number (pixels) and not a percentage string.
By default it is set to null
which means that the chart fills the entire space of the chart-container and has no fixed width.
So, what I would recommend is setting the chart-container's width to a percentage of a parent div in CSS without setting any fixed with with the chart.width
Here's a demo: https://jsfiddle.net/BlackLabel/getw6L3x/
If you want chart properties to change depending on the chart's width take a look at the responsive
property here - https://api.highcharts.com/highcharts/responsive
Let me know if that helps.