Marked or indented, you are still messing with the box model and resizing the actual span. This means that line wrappers will not appear in the right place and may violate valid fields.
You can use the after selector to add some content and style it:
your_css_class:after { content:" "; word-spacing:1em; }
I do not think that this can be done as an inline style, it needs to be done in the style of a <style> block or external file.
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