Readable doesn’t mean legible, although the two concepts share the same root: the facility with which a text may be read. Legibility has to do with recognising individual characters in a system of writing. Readability is about understanding the text, irrespective of script.
A text cannot be readable if it is not legible.
There are over a dozen tests for assessing the readability of texts. There are no tests for assessing legibility.
A modern reader is concerned with readability only. A historian dealing with sources has to worry about both. Palaeography is the study of old handwriting, the science of legibility. What the printing press has done was to remove the question of legibility, focusing all the attention on readability. For texts written before the age of print, legibility often bars the way to readability. To understand Chaucer, one has to overcome the script barrier. To enjoy the legends of King Arthur, one must first untie the legibility knot. That is not always easy.
We have no excuse for not reading a modern text because it has become illegible. The printing press is the great leveller, the only perfectly democratic institution ever founded. And also the steadiest. There is no legibility distinction between a 16th-century and a 21st-century printed text. On the other hand, a 12th-century scribe may find it hard and sometimes impossible to work out a text written in the 7th century.
Digital humanities historians and scientists are working hard to train computers to decode old handwriting, the script in manuscripts. The tunnel is long and the light is dim. The famous Flesch–Kincaid readability test gives a score for how difficult a piece of text is to understand, according to a neat formula. Legibility escapes measurement, at least for now.